Traditional plastering is a diminishing trade. The skills required to repair three-coat plaster in a pre-war home, or to apply a veneer plaster system that matches the look and feel of the original, are genuinely rare — and genuinely different from drywall installation and finishing. Hiring a drywall contractor to repair plaster in a heritage home, or treating all “plaster-like finishes” as interchangeable, leads to visible, costly failures. This guide covers what to ask when plaster — not drywall — is what you actually need.
“Traditional plaster repair is one of the most specialized skills in the trade — and one of the most frequently botched by contractors who don’t have it. The key indicators are the suction test, the mix design, and the scratch pattern. A contractor who can’t explain those three things in detail has not actually repaired three-coat plaster before. Patch it with drywall compound and it looks wrong within a year.”
— Declan Walsh, drywall finishing specialist and construction quality consultant with 20 years of experience in residential and commercial finishing across the Toronto region
Table of Contents
Understanding the Plaster Systems
What type of plaster does my home have, and what type of repair does it need?
Before any work begins, identify what you’re actually dealing with:
Three-coat plaster over metal or wood lath:
The traditional plaster system used in homes built before approximately 1950. Consists of a scratch coat, a brown coat, and a finish coat applied over a lath substrate (strips of wood or metal). Very durable when intact. Repairs require matching the original mix and application technique — not just filling with joint compound.
Two-coat plaster over gypsum lath (rock lath):
Common in homes built from the 1930s through the 1960s. Similar to three-coat but applied over small gypsum board panels rather than wood strips. Often repaired by removing the damaged section and patching with new drywall, then skim coating to match.
Veneer plaster over gypsum base (blueboard):
A thin (1/16″–3/32″) plaster coat applied over a specialized gypsum board substrate. Used in residential construction from the 1970s through today as a premium alternative to drywall finishing. Harder and more impact-resistant than joint compound. Requires specific products and techniques.
EIFS (Exterior Insulation and Finish System):
An exterior cladding system, not interior plaster — but sometimes confused. Requires specialized repair contractors.
Ask your contractor specifically which type of plaster you have and what repair approach they’re proposing.
Questions About Traditional Plaster Repair
Have you repaired three-coat plaster before, and how recently?
This is a non-negotiable qualifying question. Traditional plaster repair requires specific skills — understanding mix ratios, scratch patterns, suction rates, and drying times — that most drywall contractors don’t have. Ask specifically how many traditional plaster repair projects the contractor has completed in the last two years and ask to speak to references from those specific jobs.
What mix are you using for the scratch and brown coats?
Traditional plaster is mixed from gypsum plaster (or lime putty) with aggregate (sand) and sometimes lime. The mix ratio, aggregate size, and application technique determine the performance of the repair. Ask the contractor to describe their mix design for each coat. A contractor who responds with specifics — “we use gauging plaster with lime putty for the finish, sand-float for the brown coat” — knows what they’re doing. A vague answer is a red flag.
How will the repair be keyed to the existing lath?
Traditional plaster keys into the lath substrate — it squeezes through the gaps between lath strips and forms a mechanical bond. A repair that doesn’t properly key to the existing lath will eventually separate. Ask how the repair will be bonded to the existing substrate.
How will you match the existing texture and finish?
Traditional plaster finishes vary — smooth trowel finish, sand float, skip trowel, lime wash. Ask specifically how the repair finish will be matched to the existing plaster, and ask to see examples from previous work.
Questions About Veneer Plaster Systems
Are you trained or certified for the veneer plaster system being used?
Veneer plaster systems (USG Diamond, Structo-Base, Georgia-Pacific Goldband) require specific products, application tools, and techniques. Ask whether the contractor has manufacturer training or field experience specifically with the system being proposed.
What base board is being used with the veneer plaster?
Veneer plaster must be applied over an appropriate gypsum base board — “blueboard” is the common term, but specific products vary by system. Standard drywall is not an appropriate substrate for veneer plaster. Ask specifically what base board is being specified.
How thick will the veneer plaster coat be?
Veneer plaster is typically applied in one or two thin coats — 1/16″–3/32″ total. Ask the contractor what thickness is being applied and whether it matches the system’s specification.
How will joints in the base board be handled?
Joints in the blueboard substrate must be reinforced with fiberglass mesh tape before plaster is applied. Unreinforced joints in the substrate will telegraph through the plaster as cracks. Ask how joints are being handled.
Questions About Decorative Plaster
Do you have experience with decorative plaster finishes?
Venetian plaster, tadelakt, micro-cement, and other decorative plaster finishes are artistic applications that require specific product knowledge and significant practice. Not every plaster contractor has experience with these — and the results of attempting them without proper training are often expensive to remediate. Ask specifically about experience with the decorative finish you’re considering, and ask to see completed examples.
Related Questions
- Questions to Ask a Drywall or Plastering Contractor Before You Hire One
- Questions to Ask a Drywall Contractor About Finish Levels
- Questions to Ask a Drywall Contractor About Materials
- Red Flags When Hiring a Drywall or Plastering Contractor
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