I do a lot of drywall patches and I use a lot of five minute and twenty minute setting compound. Occasionally I get a wall that has paint, maybe enamel, I am pretty sure it is latex or acrylic and the compound I apply on the existing wall does not set in the stated time. The mud in the pan and the mud on the new gypsum panel sets like it is supposed to. So why does the mud not set over paint sometimes and what can I do about it?

 

That's not too surprising. Even though it sets chemically, it still contains water that has to dry, and the paint is a class iii vapor retarder. You could try an accelerator... a teaspoon of alum or cream of tartar in each pan you mix up will usually help it set a little faster. Or put a fan on it or use a blow dryer or heat gun to speed it up. But watch the heat gun it will blister the paint quick. Cold exterior walls can be slow to set and dry.

 

Yes, alum or cream of tartar will set gypsum products. Actually we plasterers know a lot of other ways to hurry up the set of plaster. I don't know what is in quick set mud but I am pretty sure the main active ingredient is plaster. I don't need it to set faster than five minutes and it sets just fine in the pan so moisture absorbtion or evaporation is not an issue with the set. I do use the hair dryer trick. But that takes a little time. I wish I could get the whole area to set as quickly as the set in the field of the patch or the pan. I probably use more 5-minute mud than 20-minute and rarely use anything slower.

Here is another trick: I do a lot of patches in the neighborhood or 2' square and smaller. For ones that small I install new gypsum board and give the whole thing a coat of FibaFuse. I find that a lot faster and neater than taping four joints. The FibaFuse somehow absorbs the moisture or lets the material bond with it. I have a friend who does not use setting mud and can recoat the FF in a few minutes. I guess I like setting mud because it is more like plaster and I grew up plastering. Anyway to do a patch really quickly coat the whole thing with FibaFuse in 5- minute mud then finish with another coat and texture. IF THE MUD SETS ON THE SURROUNDING PAINT, this is really quick

 

A wall well painted with an enamel will cause the mud to set slower. Is this an ongoing issue or just with your last bag of setting compound?

 

No, this is not a new issue. I just wondered if anyone knows why it happens and what to do about it. On the most recent wall, I think the paint was at least a year or so old., maybe older. I have had the same thing happen in well cured paint, but not all paint. And I am not sure but I think it is no better or worse with multiple coats, that is, in old houses that have been painted several times.

 

Flat latex paint [even multiple coats] can absorb a small amount of moisture, latex enamels usually do not. Other than that I don't have a clue with what you are experiencing.