Overcoating: The Old Coating Gets a Vote

Wally Bates

Sep 26

2 min read

Most steel in service has already been painted, so a large share of maintenance work means putting a new coating over an old one. Overcoating comes with one rule: the new coating is only as good as the old coating's adhesion to the steel.

Water tower under containment during maintenance repainting

Why it can go wrong

The new coat bonds to the old coat. If the old coat is chalked, loose or barely holding on, the new coat adds weight and stress to a system that is already failing. A strong new coating also shrinks as it cures and can pull a weak old coating off the surface. When that happens, the new product usually gets the blame.

How to assess the existing coating

Adhesion testing (ASTM D3359 tape or ASTM D4541 pull-off) shows whether the old coating is sound.
A solvent rub shows whether the new coating's solvents will soften or lift it.
A test patch, left a week or two and then tested, is the best predictor of success.

Tnemec Technical Bulletin 98-10 R2 turns this into a score. Rate the old coating's adhesion at three locations and score the delamination risk from 1 to 5. At 4.5 or higher, the recommendation is not to overcoat.

Preparing a sound surface

When the assessment supports overcoating, preparation still matters: remove chalk, wash off salts and contaminants, feather the edges of bare areas and give the old coating some profile so the new coat can grip.

The payoff

Done correctly, overcoating can extend service life at a fraction of the cost of full removal. Done without testing, it can cost the owner two coating systems.

What to check in your spec

Adhesion testing at multiple representative locations.
A compatibility test patch with a defined waiting period.
A decision rule for when full removal is required.
Cleaning and spot prep requirements (SSPC-SP 7 or SP 14 where needed).

Standards: SSPC-TU 3, SSPC-PA Guide 4, ASTM D3359, ASTM D4541, SSPC-SP 7, SSPC-SP 14.

Talk to a consultant. Atlantic Coating Consultants helps owners, engineers and contractors in PA, NJ, DE, MD, DC, WV and OH with specification writing, on-site condition assessment, training, product recommendations and field support. Call 484-644-5303 or email atlantic@tnemec.com.