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Discussion Starter · #1 ·
Doing a basement finish. I am painting nine Masonite solid core doors, all were factory primed. The factory primer was fairly smooth so I scuff sanded them to get rid of any dry spray, obvious rough spots, dust nibs etc before priming. I did not do a full sand, though in retrospective I should have. Doors were vacuumed and cleaned with a microfiber cloth before painting

I sprayed Diamond Vogel Mil-Max water based enamel primer with a Graco 390 and 410 FFLP tip. Wet film thickness was at spec at 4 mils. After I primed and was sanding the primer I started seeing lots of spots where the primer sanded through and it looked like a short hair had been under the primer. At first I assumed I had a bad batch of the Diamond Vogel primer but I looked at a couple other doors we had rejected due to poorly cut mortices that had never been sanded or primed and I could see the same thing in those when using a really bright light.

I am attaching a picture of one of the doors after sanding. Some of the spots are deep enough to feel and would need glazing putty. Does this look like bad factory primer? Any suggestions? Thanks for any help. I appreciate it very much.

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I don't normally sand [other than the ends] or prime preprimed masonite doors.
A light sanding and a coat of paint should fix it.
 

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Discussion Starter · #3 ·
I don't normally sand [other than the ends] or prime preprimed masonite doors.
A light sanding and a coat of paint should fix it.
Mark, thanks for the reply. As I mentioned some of these are deep enough to feel with a fingernail. I don't think that even two coats would fill those. I am also concerned since I have primer coming off the door in these spots, are other parts of the factory primer going to fail at some point too.

Again, thanks!
 

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Mark, thanks for the reply. As I mentioned some of these are deep enough to feel with a fingernail. I don't think that even two coats would fill those. I am also concerned since I have primer coming off the door in these spots, are other parts of the factory primer going to fail at some point too.

Again, thanks!
I would not try to sand masonite - it always ends up this way. As to failed primer, keep in mind that primer is not like paint in that it is not at all durable when exposed. It is made to be painted over. Probably nothing wrong with the original primer. As posted, a coat of paint will likely fix. Masonite is truly a horrible product.
 

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Discussion Starter · #5 · (Edited)
I would not try to sand masonite - it always ends up this way. As to failed primer, keep in mind that primer is not like paint in that it is not at all durable when exposed. It is made to be painted over. Probably nothing wrong with the original primer. As posted, a coat of paint will likely fix. Masonite is truly a horrible product.
I did spray a couple doors with primer and two coats of finish and the things that look like hairs are very visible, especially with satin. Here is zoomed in picture of of one of a slab with just the factory primer on it. It is covered in these spots. If you try to sand them at all, the factory primer just pulls off the door in those spots like in my first picture and you are down to the bare masonite.

I am going to have to sand them all with 320 and fill the ones I can feel with glazing compound. :mad:

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I had a whole huge house full of those once. Best thing to do is reprime with an oil based primer. Whats happening is that any waterborne you put on there will make the MDF fibers swell up, and youre back to square one. I dont think its so much as a bad factory prime, but bad MDF to begin with.
 

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Discussion Starter · #7 ·
I had a whole huge house full of those once. Best thing to do is reprime with an oil based primer. Whats happening is that any waterborne you put on there will make the MDF fibers swell up, and youre back to square one. I dont think its so much as a bad factory prime, but bad MDF to begin with.
Woodco,

Thanks for your reply. I just read your long door thread on over at PaintTalk. I looked at your pics and it looks much like mine do. It sounds like your main issue at the time was the way the door looked after painting. My problem (besides the crappy doors) is that the fibers are already raised up in the the factory primer with no other coating on it. The factory primer has so much orange peel and dust nibs in it there is no way I can get by without sanding these. When I even lightly sand, the factory primer just pops off the door in those spots where the fibers are raised up. Once that happens, that spot needs to be filled.

I tried using bondo glazing putty and after curing, when I lightly sand it, the bondo just pops out of those spots too. So I shot some 123 over the spots, refilled it, sanded and the 123 and bondo just pulled right out of the spot. I shot some BIN out of a rattle can on some of the spots last night and will try filling them again today and see if the bondo sticks this time. Since I only have one sprayer (490) I don't really want to spray BIN or another oil based primer with it, not to mention the fumes.

The other thing I am worried about is general adhesion of the factory primer. I don't want the paint to start flaking off the doors the first time the door gets bumped by something.

Funny thing is I have sprayed a lot of masonite doors and mdf in the past and never had any issues like this.

Thanks again.
 

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Maybe try filling with MH Ready Patch. Its an alkyd oil based filler which should adhere to MDF and Hardboard same as an oil based primer would. It’d probably require 2 thin applications.

I like to sand and reprime hardboard with a high build alkyd undercoater such as Moore’s 217 alkyd enamel underbody..
 

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Discussion Starter · #9 ·
Maybe try filling with MH Ready Patch. Its an alkyd oil based filler which should adhere to MDF and Hardboard same as an oil based primer would. It’d probably require 2 thin applications.

I like to sand and reprime hardboard with a high build alkyd undercoater such as Moore’s 217 alkyd enamel underbody..
Thanks. If my filler doesn't stick to the BIN I may try this. Thanks for the recommendation!
 
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