Mystery Spots on Finished MDF

Finishers try to figure out why pale pink spots keep materializing through opaque finishes on MDF panels. November 25, 2008

We do two color/painted finishes. They both are a tinted base coat of pigment lacquer. One has a vinyl seal with clear conversion varnish top. The other has nitro sand seal with clear nitro water white finish. Spray is with AAA 10:1 with Kremlin MVX gun. In both cases during the clear topcoats, we have a soft pink spot that appears seemingly under the clear finish. It won't sand out and occasionally requires touchup. Our AAA system only sprays the clear coats and the lines have never had any other finish in them.

Forum Responses
(Finishing Forum)
From contributor G:
Something on someone's hands, a machine gripper or something else the spot comes in contact with (vacuum lifter)?

From the original questioner:
This happened yesterday and I was doing the spraying. I had new nitrile gloves on that were clean and after my pass with the clear finish, the soft pink spot appeared. It is always pink and always a soft spot. Nothing that I know of in our gun system is pink. The only thing that is put in is Kremlin grease on the inner parts, but it is not pink.

From contributor S:
A mystery for sure. I would consider a conversion varnish over a pigmented lacquer to be a no-no, on multiple grounds, including the acid catalyst reacting with the pigment. But with the same thing happening on a nitro over nitro, you have something else going on.
I'm stumped.

From the original questioner:
The vinyl sealer is the barrier between the nitro sealer and the conversion. We've used it for several years with no failure problems. (If the vinyl sealer is forgotten, the results are terrible!)

From contributor S:
I understand that you are successfully using the vinyl seal as a barrier to bridge the chemistry, but you still are applying a harder over a softer finish. Which is not where you are having a problem at this point. And I am still stumped regarding your pink problem.

From contributor J:
I am 99.999% sure that it's the wood filler you are using. What kind are you using? I have found this happens mostly with solvent based fillers.

From the original questioner:
In every case, this pink mystery appears on MDF panels that have been painted with the colored lacquer base and topcoated. There is no filler used. I wonder if it is a small chemical reaction. The only strange thing is that it is only one spot in large multiples of doors or panels.

From contributor S:
There is little doubt that it is a chemical reaction and now you are beginning to provide some of the necessary info. First let's clear up one thing - when you used the word "soft," were you referring to the color pink being soft as in pastel, or is the finish coating over that area soft on a hardness basis?

Did the MDF come in as sheet goods which you then manufactured into doors, etc., or did you outsource and these doors came in ready to paint?

Either way, you can do some forensics to isolate the problem. For the heck of it, check the product out with a UV lamp (black light) to see if it fluoresces. If you can, check out pieces before paint, after paint, and after clear coat. If it fluoresces, you just got a shot at containing it before it shows up after final coat. A quick shot of Bin pigmented shellac prior to your lacquer coat may isolate it and prevent it from bleeding through.

From the original questioner:
Soft as in pastel. The black light is interesting.

The photo shows the trim piece with the pink spot on it. The brown below is the background.

Click here for higher quality, full size image

From contributor N:
I've seen dyes climb through several layers of CV. Is there any chance there might have been some dye splashed on the MDF? Is it stored near the stain mixing department at your distributor? Is there dye in the tinted lacquer basecoat? Another thought - is there any chance catalyst got splashed on it? Perhaps you could rework the finishing schedules to using more compatible products. There are better basecoats than tinted lacquer.

From the original questioner:
The base coat is white lacquer undercoater that is mixed to match by the paint supplier. The weird thing is it only happens during final clear coats. I'm open to other base products.

From contributor D:
Lacquer doesn't make sense for a number of reasons. Use the MDF specific undercoater (Valspar's word). This is superior for a lot of reasons.

1) It's an acid catalyzed product just like the topcoat. What I see here is a reaction to the acid in the topcoat.

2) It has three times the solids content as the lacquer.

3) These undercoaters have a lot of talc in them to fill the routed MDF pores and make sanding easier.

Go with this all CV system. The final price is about the same as the solids of the CV make up for the higher unit price per gallon.

From contributor E:
It's a contamination and not really a chemical compatibility issue. Wash down the substrates with lacquer thinner before doing any finishing. I think the problem begins with the substrate. As your topcoat is applied, something causes this color either to separate out or to be formed, and it appears in its spot as a spot. This color could be in the mix as a trace color. Or it could be formed. If the substrate were perfectly cleaned and then not handled without clean gloves, you have your best shot at finishing without spots forming. That's my polar guess.

From contributor M:
I have had this exact thing happen with MDF, except my problem was blue. Here is what you need to do: scour with your eyes over your next order of MDF doors. Look for absolutely any chalky red substance on the surface. I've seen it red and blue. The blue got me - the color leached up through all the sealer and lacquer and caused wailing and gnashing of teeth. It also caused a bit of fisheye if I tried to clean it off with lacquer thinner.

I do not know what the heck the stuff is. Sometimes it looks like a paint brush stroked across the MDF, sometimes it was just small spots. But it does cause color spots. Now if you can check the doors and find those red spots before you spray, contact your supplier and kindly tell them of your problem and that you'll refuse any MDF that even smells like it came close to the red stuff.

If you do not find red spots that become pink spots... well, beats me!

From the original questioner:
Thanks. I will have the guys begin to look more thoroughly at the MDF as we work the products.