We have been discussing touching up catalyzed finishes. In the course of my research, I sent an e-mail to one of my buddies to ask for some background on the use of MEK and PM Acetate for repairing finishes. My buddy Kevin Kamberg is an expert in doing this and he responded with words that were well written. I have decided to use him as a guest writer today. With his permission I have edited for clarity but otherwise let him tell you his methods in his own words. Here goes!
A Run or Sag
First, I would never try to just sand out a run. Likewise, unless a sag is really minor, I wouldn't try to just sand that either. That goes double for dry sanding! Wet sanding isn't as problematic because the lubricating liquid (usually water or mineral spirits depending on the finish) concentrates the abrasive on just the high points. Dry sanding almost always produces a non-level surface. Wet sanding to any meaningful degree does too but less so in my experience.
I always want to shave off as much of the run or sag as I can. A veneer scraper works very well but a single-sided razor blade (the thinner the blade the better) works well enough that I just about always reach for that. I use the razor blade in the same manner as I would a veneer scraper, pulling off shavings rather than trying to cut them off. Cutting them off can be very dicey. Pulling off shavings is far safer. I hold the blade at about 80 degrees, so that the cutting edge is trailing behind the edge being held. I use light pressure at first as I begin dragging it across the run or sag, trying to keep that spot in the middle of the blade so that the surrounding level surface acts as a guide or indicator.
I remove as much as possible and then, depending on how perfect I need the fixed surface to be, I either dry sand or wet sand the remainder.
You'd think that a sanding block would be helpful but I find that the edges often gouge too deeply, creating a bigger problems. I use finger pressure to localize the sanding to just the run/sag and the immediately surrounding area. 320 grit is my strong preference. 220 can work but it’s too easy to burn through the lacquer. 400 grit and finer can leave a polished surface that doesn't resemble the surrounding finish.
The next step in the repair is to use a blend of a solvents; (I prefer MEKCAS No. 78-93-3but Acetone 67-64-1works too)combined with a lacquer retarder (I'm fond of P.M. Acetate 108-65-6but others may work just as well), I mist over the repaired surface, using multiple light coats to slowly“sneak up” on the repair. The goal is to rewet the surface just enough to flow the repaired area back into the surrounding area.
My ration could go up to 50/50 in hot weather. But, I have no doubt that 25% MEK and 75% PMA will work well on almost all occasions.
Using this minimalist approach also helps keep the flatting agent in the lacquer in an appropriate solution so that the repair looks like the surrounding lacquer. The hot solvent (MEK or Acetone) bites in and dissolves the surface but then quickly flashes off so that it doesn't bite any deeper into the finish than necessary. The retarder (PM Acetate) keeps the surface open long enough for things to reflow properly.
Straight MEK can be used but it flashes so quickly that it can be hard to get the surface to reflow right without using too much. Obviously, Acetone would be even more problematic. The retarder is what makes thingscome together in the end.
With no-catalyzed lacquer, (CAB, nitro/alkyd hybrids) dryer is better in terms of how cured the surface is before the repair is done. If the offending run/sag is too green then trying to shave it off can end up pulling lacquer all the way down to the wood’s surface. Be careful here as you may create a much bigger problem. However, I find that the lacquer reflows best if the repair is done the same day.
With pre-cat lacquers same day repairs are essential. Because of the catalyst, only part of the surface will reflow if the repair is done the next day. So I always, always, always want to fix a pre-cat sag the same day, and sooner is preferable to later...as long as the lacquer is dry enough to shave with the above technique. If the run or sag is in a low visibility area then a next day repair may pass muster but definitely not in a high visibility area.
If it is necessary to repair a run or sag in pre-cat the next day or later, just lightly spray the repair with fresh lacquer and then use the mist technique to flow it and any overspray back into the original surface. However you will need to clean the surface first if it's an old surface.
Next week we will continue on with Kevin’s discussion of repairing solvent-based lacquers with a combination of MEK and PM Acetate. Please stay tuned because he had more helpful tips to offer.
Until next time…spray on!