Troubleshooting Finish Chipping

A finisher asks for help with top-coat chipping of catalyzed finishes. A change in sanding methods turns out to be the culprit. August 30, 2005

I am having problems of late with the finish on our Kitchen cab doors (MDF). I am using what we have always used and as far as I am aware have made no changes in mix, but the paint is chipping real bad.

The finish is two coats of Beckerís white surfacer followed by the topcoats Beckers two part 10/1 hardener and I tend to thin by about 25%. By the time the doors get to the job they are chopped to buggery, even when they are laid face to face with paper between they will tend to scratch. Any little knocks during install and they chip not just the edge but also on the main flat surfaces as well. Itís driving me mad and profits are down. Does anyone have ideas or advice?

Forum Responses
(Finishing Forum)
From contributor G:
As I recall, the catalyst requirement for Unisurfacer is 13% and for Matador is 12%.

From contributor R:
Have you changed the type of MDF, or has your supplier changed their source for MDF?

From contributor D:
If it is not your catalyzing ratio that's off then I suggest that you have a problem with inter-coat adhesion. What is your sanding and scuffing schedule? Also, what is the temperature range while the items are curing? Maybe they cure at too low a temperature. Those are the things that cause finish failures of your type, assuming that the finish is failing and not that these are just damages to the finish.

From contributor S:
It could be inter-coat adhesion if the finish is chipping between coats. If the chipping is going all the way down to expose the MDF then you have another problem.

From the original questioner:
What is everyoneís sanding and scuffing schedule? Thatís a good point because its now occurred to me that we have made some changes of late. We started using different materials for sanding and the final item now used between the surfacer and topcoats is a pad type sander fine grade, a bit like a pan cleaning pad. This could be causing adhesion problems for the topcoats, with the fact that itís now winter in Australia.

Your other comment Contributor D - "what is the temperature range while the items are curing? Maybe they cure at too low a temperature" could also be a problem. The curing must be taking longer no doubts on that, but the guys are still spraying the same and often due to the workload are laying two heavy topcoats one after the other.

SH mentions if the chipping is going all the way down to the MDF then itís another problem. Itís not going all the way down, itís always the topcoat chipping away from the surfacer.

From contributor P:
The catalyst ratio for the Unisurfacer is 12% by volume, 10% by weight. As far as topcoats, I have used the Matador White and the Bernyl Strong White. Matador is catalyzed exactly the same as the Unisrufacer. The Bernyl Strong is catalyzed 16% by volume (not sure about weight, but it's probably about 12%). This being said, it sounds like a sanding issue.

From contributor S:
Catalyzed finishes should be scuff sanded just prior to applying the next coat of finish. If you are scuff sanding the day before or sanding too fine then that could risk good inter-coat adhesion. Create a scratch pattern by sanding just prior to applying another coat of finish.

From contributor N:
I am not familiar with these particular products, but I can tell you that if the topcoat is a 2K urethane that urethanes shut down catalyzation completely at 50F. This is absolutely true for automotive clearcoats and I see no reason why 2K wood coatings would be any different as the isocyanate chemistry is identical.

However the sanding seems more likely the issue if the temperature is above 65F. Anything finer than P320 is unnecessary and undesirable. Catalyzed coatings do not melt into each other after the chemical reaction is completed. They only cling by a mechanical bond you must achieve using abrasion.

Urethanes can be applied wet on wet (this is one of their great advantages in automotive refinishing), but if the primer is allowed to cure prior to basecoat application (cure occurs overnight in the automotive world) the primer must be abraded.

From the original questioner:
I am sure this is a problem more of adhesion than anything else. Our new pad sanding system is producing a finer result than 320. I looked at two samples using a microscope, the pad is tending to smooth the top from the cut, and the paper is not doing this. The cuts remain deeper and more pointed in form which yields better adhesion.

I then used paper and pad of the same or sim grade and the same thing occurred. The paper is cutting better and the pad is scratching, but then also smoothing the top off because of the way itís structured.