Painting problems

[edekoning]

I started painting my MDF cab. I did a ton of sanding and then applied a first layer of a special MDF primer (water based). The next day I checked the results, and all the surfaces were really rough. I noticed that a lot of MDF fibers were visible and feelable. I'm pretty sure this was not the case before I started painting. I sanded the two sides and took two pictures of the result. You really need to look at them full size. Notice that several MDF fibers are clearly visible. The surface is a lot smoother now, but all these fibers are still very feelable (is that even a word?).

So my questions are:
-Is this normal when painting MDF?
-Should I keep on sanding until its smooth again? I'm afraid I'll just sand of most of the primer, and whose to say it won't happen again when I apply a new layer of primer.
-Since almost my entire cab will be covered in vinyl and I don't really care about the inside that much, does it even matter? Meaning, will the vinyl not just cover this up, or will it be visible through the vinyl?

[jennifer]

     The primer should cover, As long as its not all chunky in the can, (That can happen if its been frozen). I would sand it flat, reapply,
sand again, using about a 400grit... It kinda looks in those pics that the resin in the mdf is trying to repel the product. It should cover
however. It needs to be flat before you put the vinyl on every little bump will be accenuated,...Btw...Dont sand that coat off, just
knock the high spots down, try not to get back into the wood, It may take a few applications.

[Woodshop Flunky]

Sounds like raising the grain.  But I've never seen this before on MDF.

The other thing that comes to mind right off, is fibers in your shop that "settled".  Couldn't know from just the photos.

Either way, I think you could lightly sand the sides to knock the texture back down.  You might not even need a second coat.  I usually sand after primer coats anyway.

[dfmaverick]

Did you wipe down the MDF after sanding with a tack cloth?

[edekoning]

I don't have a shop/garage I only have my garden for sawing, and my computer/game room for doing the rest (i.e.: painting).

I used special MDF cleaning tissues, then sanded everything, used a vacuum cleaner, brushed off the entire cab with a soft brush, let the dust settle for a day, vacuumed again, then painted, waited a day to check the results. One thing I did notice after using the cleaning tissues was that the MDF surface was no longer as smooth, but felt a bit coarse. I did sand it all smooth again though. Also after the primer dried the entire cab felt like rough sanding paper. I sanded the entire cab until it was either smooth, or until I started removing the primer. Took me almost 4 hours, pfff. Anyway it really needed another layer of primer, which I applied last weekend. I spend an hour or so last night sanding the sides and front. The result feels a lot smoother overall and the second coat covered really well. Hopefully I'll have some time this weekend to put on the first layer of black paint.

[Sjaak]

I don't have a shop/garage I only have my garden for sawing, and my computer/game room for doing the rest (i.e.: painting).

I used special MDF cleaning tissues, then sanded everything, used a vacuum cleaner, brushed off the entire cab with a soft brush, let the dust settle for a day, vacuumed again, then painted, waited a day to check the results. One thing I did notice after using the cleaning tissues was that the MDF surface was no longer as smooth, but felt a bit coarse. I did sand it all smooth again though. Also after the primer dried the entire cab felt like rough sanding paper. I sanded the entire cab until it was either smooth, or until I started removing the primer. Took me almost 4 hours, pfff. Anyway it really needed another layer of primer, which I applied last weekend. I spend an hour or so last night sanding the sides and front. The result feels a lot smoother overall and the second coat covered really well. Hopefully I'll have some time this weekend to put on the first layer of black paint.

When painting MDF I use at least two coats of primer. After the first coat, the mdf feels worse (hairy) than before the first coat. After the second (or third) coat it's much better. And then paint at least 2 layers of color on top of that.

And never use water based paint (acryl) because it dries to fast and you keep seeing brush strokes or roller marks. It's better to use an alkyd based paint.

[kahlid74]

When painting MDF I use at least two coats of primer. After the first coat, the mdf feels worse (hairy) than before the first coat. After the second (or third) coat it's much better. And then paint at least 2 layers of color on top of that.

And never use water based paint (acryl) because it dries to fast and you keep seeing brush strokes or roller marks. It's better to use an alkyd based paint.

This has been my experience to the T.  Two coats always and no water based primers.

[edekoning]

And never use water based paint (acryl) because it dries to fast and you keep seeing brush strokes or roller marks. It's better to use an alkyd based paint.

This has been my experience to the T.  Two coats always and no water based primers.

Hmm, the MDF primer I used is actually water based. So I guess that's part of the problem.