Time Frame for Scuff Sanding and Topcoat Application

It turns out that there are physical reasons why manufacturers recommend no overnight waiting between sanding and applying a second coat of a catalyzed finish. Here's a detailed explanation. January 2, 2012

My sales rep says you have to sand the seal coat the same day as you topcoat, because if you sand the day before, then top the next day, you could run into adhesion problems. I use both conversion sealer and topcoat, so maybe that makes it more critical. I just don't see how it would matter that much. If the sealer is sanded and roughed up, why would there be an adhesion problem by waiting a day or two to topcoat?

Forum Responses
(Finishing Forum)
From contributor J:
My MLC rep said the same thing. He said changes in humidity may cause it to swell and some of the scratching may close up a bit. No idea if it's valid, but that's what he said.

From contributor B:
I've never heard that, and I've been in the business 21 years. As far as I can tell, the only thing that happens if you sand one day and recoat the next is getting a little additional dust on your substrate from sitting around. Shops all over the nation seal sand and wait overnight (thinking about large automated lines that shut down at a certain point every day).

From contributor C:
Depends on the coating. Different rules for different coatings. Most coatings reps are wrong more than right and have very limited know how on finishing. We had the MLC guys in last week and they couldn't answer half my questions and gave me bad info on several.

From contributor T:
I've wondered about this myself. I sand the day I spray because that's what my rep said, but I never got a good answer as to why. Hopefully some factory rep can chime in and enlighten us.

From the original questioner:
I have had reps from 2 different companies (ML Campbell and Gemini) tell me you have to topcoat the same day as you sand. That can be very hard to do on a bigger job where you have a lot to sand. Some days it's just not possible. I would like to be assured that it's not worth worrying about.

From contributor D:
Once you mechanically abrade the surface suitable for a topcoat, only a change in the level of abrasion should make a difference in its ability to accept a topcoat. Can temperature or humidity do that? It's not likely that a finish can expand or contract that much in one day, except maybe if you have a steam leak or sudden freeze.

From contributor R:
Most reps/salespeople have very little hands on experience in the real world of wood finishing and likely are just parroting what the bean counters tell them to say. I once had a rep from a well known company tell me it was impossible to apply a nitrocellulose lacquer over an oil based varnish, and if I tried to do so, the sky would come falling down and the earth would reverse its rotation. Later on, a finisher with gobs upon gobs of actual hands on experience showed me how to do it, and I not have any issues.

That was well over 25 years ago, and I've never forgot his words of wisdom. "Make samples of what you want to do, and put them through real world testing." He showed me samples from his library that were almost as old as I was. He had not only kept his successes, but his failures as well. The recipes and procedures were well documented on the backs of each one of those samples.

My advice would be to take a weekend when it's quiet in the shop and start producing some samples. Do everything that you're not supposed to do (according to the sales people) and see what happens. Stick them up on the roof, put them in the microwave, pour various chemicals all over them, scratch them up and do a water soak test, etc. Find out for yourself exactly what works and what won't. Ronald Regan said it best, "trust, but verify."

Sure, a manufacturer's chemist does some testing but they do their testing in a perfect environment, a perfect temperature controlled room, a spotless and dust free spray booth that NASA would envy, but most of us finishers don't have the luxury of these goodies at our disposal.

I think those chemists and salespeople and so called reps could learn way more from us folks who use the products day after day than they can from sitting in sales and production meetings.

From contributor A:
The truth: if you're using evaporative finishes you can spray anytime, even weeks later, with or without sanding. With catalyzed finishes, this changes.

Fresh sanding a surface creates sharp ridges at various angles which give good tooth (sharp grooves) which the next coat can adhere well to. When the fresh sanded areas sit for hours or days, the finish is in flux and continues to change shape as it loses solvents, making the scratches less sharp in profile (the grooves and edges become microscopically rounded and less sharp). This can be seen under 60X magnification or higher if you cut a sample and look at it from the edge and compare it to one just sanded.

This happens with all cat finishes but is worse with post cat than pre. Though the rules should be followed on all. Like so many other things in the field of coatings, you may get away with it for a long time without problems, and some may never have them, at least that they know of, because customers don't tell them of a problem that happens way down the road. There are many practices that go against what the manufacturer recommends that are gotten away with, but usually as luck goes, it will show up at the least affordable time and make you wish you had listened.

For those who say their job is too big to sand and spray in one day, I suggest you sand and spray as you go - sand one door or one cabinet and spray, then continue that till the job is complete. Personally, this is common sense when you know the rules you need to follow and once you get used to it, is no more time consuming than sanding all and then spraying.

From contributor D:
Contributor A, you always have informed responses, and I respect your insights. What I'm curious about is if the finish is cured enough to sand properly, wouldn't the changes you write about be insignificant? I know certain finishes will take longer for a full cure, but if you can sand it and it dusts well, wouldn't the most dramatic change already have taken place? Also, what if you seal a piece, let's say wait a week, then sand it, then wait a day or two before the final coat - would your explanation still hold?

Must you topcoat the same day as you sand? Seems like it would depend on the degree of cure the substrate has.

From contributor L:
As long as the finish is curing, it is changing. So when you sand a finish that is still curing, you need to keep the same day finishing. Curing is a long process, a lot longer than you would think. If you over catalyze a finish, it will turn pink in about 6 months. That means a chemical reaction is still occurring. Plus, if the manufacturer says you need to sand between coats and do it the same day and you don't, your warranty is void.

From contributor Q:
Contributor A once again has the most logical explanation. The reps could also be confusing two different situations. Wood needs to be abraded as close to the actual coating time as possible. The wood starts to oxidize as soon as you bring up fresh material. They may be using that info to infer that all surfaces must be abraded in the same time frame. The other possibility is simply covering their own butts. MLC will always discourage abrasion beyond 240 grit. We all know that is pretty coarse. Depending on the finish and sheen, you can see 240 grit scratches beneath the topcoat.

From contributor S:
Read the can. It is even the same for pigments.