Replicating Ornamental Plaster Trim

Procedure code:
920005S
Source:
The Old Custom House/Ny, Ny - Gsa/Pbs, 2-27-91
Division:
Finishes
Section:
Lath & Plaster
Last Modified:
03/13/2017

 

REPLICATING ORNAMENTAL PLASTER TRIM

 

 

ORNAMENTAL PLASTER WORK IS A SKILLED CRAFT REQUIRING THE WORK OF A

QUALIFIED AND EXPERIENCED CRAFTSMAN.

 

 

PART 1---GENERAL

1.01 SUMMARY

 

    A.   This procedure includes guidance on replicating

         ornamental plaster trimwork to replace that which is

         deteriorated or missing.

 

    B.   See 01100-07-S for general project guidelines to be used

         along with this procedure.  These guidelines cover the

         following sections:

 

         1.   Historic Structures Precautions

 

         2.   Submittals

 

         3.   Quality Assurance

 

         4.   Delivery, Storage and Handling

 

         5.   Project/Site Conditions

 

         6.   Sequencing and Scheduling

 

         7.   General Protection (Surface and Surrounding)

 

         These guidelines should be reviewed prior to performing

         this procedure and should be followed along with

         recommendations from the Regional Historic Preservation

         Officer (RHPO).

 

1.02 REFERENCES

 

    A.   American Society for Testing and Materials (ASTM)

         www.astm.org

 

1.03 SUBMITTALS

 

    A.   Product Data:  Submit manufacturer's product data for

         plaster materials, lath and accessories.

 

    B.   Design Data/Test Reports/Certificates:

 

         1.   Material Certificates:  Submit producer's

              certificates for plaster aggregates to show that

              materials comply with requirements.

 

         2.   Schedule of Work:  Submit a schedule of spaces that

              require ornamental plaster, and a list of

              ornamental plaster items or areas that require

              repair or replacement in each of the affected

              spaces.  Indicate procedures to be used, and time

              requirements.

 

1.04 QUALITY ASSURANCE

 

    A.   Regulatory Requirements:  Reference Standards:  Comply

         with applicable requirements of Chapter 9 - Ornamental

         Plaster, in "Plastering Skills", published by American

         Technical Publishers, Inc.  Comply with other specified

         standards.

 

1.05 PROJECT/SITE CONDITIONS

 

    A.   Environmental Requirements:

 

         1.   General:  Comply with requirements of referenced

              plaster application standards and recommendations

              of plaster manufacturer for environmental

              conditions before, during and after application of

              plaster.

 

         2.   Cold Weather Protection:  When outdoor temperature

              is below 55 degrees F., maintain continuous uniform

              temperature of not less than 55 degrees F., nor

              more than 70 degrees F., for not less than 1 week

              prior to plaster application, during its

              application, and until plaster is dry but for not

              less than one week after application is complete.

              Distribute heat evenly; prevent concentrated or

              uneven heat from contacting plaster near heat

              source.

 

         3.   Ventilation:  Ventilate building spaces as required

              to remove water in excess of that required for

              hydration of plaster.  Begin ventilation

              immediately after plaster is applied and continue

              until it sets.

 

         4.   Protection:  Protect adjacent work from soiling,

              splattering, moisture and other harmful effects

              which might result from plastering.  Obtain

              approval of the protection from the contracting

              officer.

 

 

PART 2---PRODUCTS

2.01 MANUFACTURERS

 

    A.   National Gypsum Co.

         Gold Bond Building Products Division

         www.nationalgypsum.com 

 

    B.   USG Corporation

         www.usg.com 

 

    C.   Perma-Flex Mold Company

         www.perma-flex.com 

 

    D.   Charles Street Supply Co.

         www.charlesstsupply.com 

 

2.02 MATERIALS

 

    A.   Gypsum Plaster Materials:

 

         1.   Base coat plasters:  ASTM C28, types as indicated

              below.

 

              a.   Gypsum neat plaster such as "Red Top Gypsum

                   Plaster" (USG.com), "Two-Way Hardwall Gypsum

                   Plaster" (National Gypsum Co.), or approved

                   equal.

 

              b.   High strength gypsum neat plaster with a

                   minimum average dry compressive strength of

                   2,800 psi per ASTM C472 for a mix of 100 lbs.

                   plaster and 2 cu. ft. of sand such as

                   "Structo-Base" (USG), or approved equal.

 

         2.   Finish coat plasters:  Types as indicated below:

 

              a.   Gypsum gauging plaster, ASTM C28; for flatwork

                   such as, "Red Top Gauging Plaster" (USG), "Gold Bond Gauging

                   Plaster" (National Gypsum Co.), or approved

                   equal.

 

              b.   Gypsum molding plaster, ASTM C59; for crown

                   moldings such as "USG No. 1 Moulding Plaster" (USG)

                  "Gold Bond Moulding Plaster" (National Gypsum Co.), or

                   approved equal.

 

              c.   Gypsum casting plaster, ASTM C59; for

                   ornamental plaster such as "USG No. 1 Casting

                   Plaster" (USG), or approved equal.

 

         3.   Finishing Hydrated Limes:  ASTM C206; Type S, or approved

              equal.

 

         4.   Aggregates for Base Coat Plasters:  ASTM C35, sand.

 

    B.   Clean, potable water

 

    C.   Bonding Compound for Gypsum Plaster:  ASTM C631.

 

    D.   Plaster Molds:  Urethane rubber liquid mold material that

         can produce detailed impressions (Perma Flex Mold Co.),

         or approved equal.

 

    E.   Reinforcing Fibers:  Sisal hemp fibers, or polypropylene

         fibers, not more than 2" long (Plastic Tooling Co.), or

         approved equal.

 

    F.   Anchors for Reattachment of Existing Plaster:  Zinc

         plated washers with appropriate mechanical fasteners for

         existing substrates, (Charles Street Supply Co.), or

         approved equal.

 

2.03 MIXES

 

    A.   Plaster Base Coat Compositions:  Comply with ASTM C842

         and manufacturer's directions for gypsum plaster base

         coat proportions for three-coat work:

 

         1.   Scratch Coat:  High strength gypsum gauging plaster

              with job-mixed sand.

 

         2.   Brown Coat:  High strength gypsum gauging plaster

              with job-mixed sand.

 

    B.   For Finish Coat:  Proportion casting plaster and molding

         plaster to comply with the manufacturer's instructions.

         Proportion gypsum gauging plaster to comply with ASTM

         C842; 1 part plaster and 2 parts lime.

 

 

PART 3---EXECUTION

3.01 EXAMINATION

 

    A.   Examine surfaces to receive ornamental plaster and

         conditions under which the ornamental plaster will be

         installed.  Proceed with the ornamental plaster work only

         when substrate surfaces and conditions comply with

         referenced standards, to ensure satisfactory

         installations.

 

         1.   Promptly notify the contracting officer of

              conditions that need to be corrected or

              coordinated.

 

3.02 PREPARATION

 

    A.   Surface Preparation:

 

         1.   Remove ornamental plaster that is damaged or loose

              and requires replacement, or that has to be reset.

 

         2.   Reinforce and secure loose ornamental plaster that

              is to remain, using appropriate concealed methods

              (see 09210-04-R for guidance).

 

         3.   Make molds or screeds as necessary for replacement

              of ornamental plaster.  Match existing plaster

              profiles.

 

         4.   Make accurate reproductions for missing,

              deteriorated or damaged ornamental plaster.

 

         5.   Clean plaster surfaces to be restored, to remove

              loose and deleterious materials that may affect

              adhesion or application of new plaster.  Re-attach

              loose lath or install new lath as required.

 

         6.   Spray existing plaster to be pointed or patched

              with water, or use bonding agent, to ensure

              adhesion of new plaster.  Install keying systems

              for larger pieces as necessary.  Comply with

              referenced standards.

 

         7.   All existing plaster to be painted shall be

              stripped down to a bare finish plaster surface with

              a non-water based solvent of paint stripper which

              cannot be water flushed clean.

 

         8.   Measure and layout ornamental plaster accurately to

              maintain pattern and alignment of the work.

 

3.03 ERECTION, INSTALLATION, APPLICATION

 

    NOTE:  Conform to the requirements of the latest edition of "Gypsum Construction

    Handbook" published by USG Corporation concerning application of ornamental plaster.

 

    A.   Make a template from sheets of metal (brass) cut full-

         scale in shapes to match existing; use to screed or form

         gypsum plaster to desired shapes and contours.

 

         1.   Coat metal sheet with layout dye - a blue dye that

              makes the scribed pattern easier to follow.

 

         2.   When the dye is dry, scribe, cut and carefully file

              the template to remove any roughness or other

              imperfections that might be transferred to the

              finished work.

 

         3.   Use a sled to guide the template as it is pushed

              through the plastic mass.  Basic parts of the sled

              include a "slipper-board" and the board to which

              the template is nailed.

 

              a.   For bench work the sled is guided by the table

                   edge.  Bench-run molds are installed on the

                   job with mechanical fasteners or adhesively

                   attached with gypsum plaster.

 

              b.   For run-in-place work, a guide strip is

                   attached to the wall or ceiling and forms one

                   edge of the pattern; after work is completed

                   the guide strip is removed.  

 

         4.   For circular shapes:  

 

              a.   Follow procedures described in Section 3.03

                   A.1.- A.3. above, except scribe the center

                   line of the circular shape on the template.  

 

              b.   Center a pivot plate to this line and attach

                   to the template board.

 

    C.   Mix Plaster:  Use special plaster designed and

         manufactured for ornamental plaster work.

 

         NOTE:  Mix the plaster a little below normal consistency

         for the first mix, then allow the mix to "cream" a bit

         before placing it on the bench in front of the template.

         For run-in-place work a slightly stiffer consistency is

         desirable.

 

         1.   For Hand Mixing:

 

              a.   Weigh the plaster, and measure or weigh the

                   water.

 

              b.   Sift plaster into water evenly.  Do not drop

                   handsful of plaster into the water at one

                   time.

 

              c.   Allow to soak 2 minutes undisturbed, then mix

                   thoroughly.  Stir from bottom, forcing

                   material to top.  Take care to avoid beating

                   air into mix.  Proper mixing forces air out of

                   mix.  

 

              d.   Continue mixing until a smooth, even mass is

                   produced.

 

         2.   For Mechanical Mixing:  Mechanically mix

              cementitious and aggregate materials for plasters

              to comply with applicable referenced application

              standards and with recommendations of plasters

              manufacturer.

 

              a.   Weigh plaster and measure water accurately.  

 

              b.   Sift or strew plaster slowly and evenly.  

 

              c.   Allow it to soak 2 to 5 minutes, and mix for 2

                   to 5 minutes.  

 

              NOTE:  For batches of 10 to 50 lbs., use a 3"

              diameter, 3-blade, 25 degree pitch propeller with

              1/4 or 1/3 hp. 1,760 rpm direct-drive motor.  For

              batches of 50 to 200 lb., use a 4" diameter, 3-

              blade, 25 degree pitch propeller with 1/2 hp. 1,760

              rpm direct-drive motor.

 

    D.   Replicate the Ornamental Trim:

 

         1.   For Run Work or Straight Mold:

 

              NOTE:  This method is used to produce staff work or

              pattern stock having parallel edges or to form a

              straight molding.

 

              a.   For bench work, some plaster should be allowed

                   to lap over the end of the bench to act as an

                   anchor and prevent the mold from loosening

                   from the bench before work is completed.  

 

              b.   Press small lumps of modeling clay onto the

                   bench before the plaster is placed provide a

                   grip for the plaster and help hold it in

                   place.

 

         2.   For Circular Shapes (Turning):  Like run work,

              turning can be done in the shop or run-in-place.

 

              a.   For bench work, place a center post in a hole

                   in the bench.  

 

              b.   For run-in-place work, position the center

                   post accurately at the center of the circular

                   shape to be formed and securely attach or

                   brace to the structural supports.  

 

                   NOTE:  THE CENTER POST SHOULD HAVE A RUNNING

                   THREAD, WITH A DOUBLE NUT FOR HEIGHT

                   ADJUSTMENT.  THE PIVOT POINT MUST BE

                   ABSOLUTELY RIGID AND SET HIGHER THAN THE TOP

                   OF THE PATTERN TO BE MADE, SO THAT THE

                   TEMPLATE WILL SCREED CLEARLY AS IT ROTATES

                   AROUND THE PIVOT POINT.

 

         3.   Duplicate plaster molds by taking impressions from

              existing plaster molds; create new rubber molds

              from these impressions and cast plaster into these

              molds.  

 

         4.   Remove plaster molds after plaster has cured,

              following plaster manufacturer's instructions.

 

         5.   Strip and clean existing decoration to produce a

              clean, sharp mold.

 

3.04 ADJUSTING/CLEANING

 

    A.   When replication and plasterwork is completed, remove

         unused materials, containers and equipment.  Clean floors

         and adjacent surfaces of all plaster debris, including

         plaster droppings daily.

 

    B.   Provide final protection and maintain conditions, in a

         manner suitable to the RHPO, which ensure ornamental

         plaster being without damage or deterioration at time of

         substantial completion.

 

                         END OF SECTION

 

Last Reviewed 2017-03-13