High Build Primer sanding

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PostPosted: Wed Feb 13, 2019 4:49 pm
What are the pieces from, what are they? your probably going to have to give us a fairly detailed run down of what you do from start to finish also a list of your materials like 2k primer, name and the activator.
Jay D.
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PostPosted: Thu Feb 14, 2019 6:18 am
Thanks Jay D.

Ok. So heres the run down:

Living in England. Its cold here right now. about 55 degrees inside my shop. I have a gas heater in the shop which I try run for 30 min before spraying to get the temp up to 60 degrees. I warm all my coatings up by sitting them on top of a electric oil-filled heater, and get them up to 95f.

I have an extractor I built with a booth filter to extract the overspray which I run in the shop venting outside.

I'm currently spraying an outboard motor, the cover cowlings. They're made of fibreglass. The paint on them was original from factory, and was a 2-stage base/clear (verified by some of the peeling clear). I degreased them very well with a water-based degreaser and scrub brush and pressure washed them, then sanded with 180 grit using a 6" DA sander with foam interface pad and by hand using a soft block. Masked the insides, wiped with Silicon Remover (bought from autobody paint supplier), using kitchen paper towels.

Sprayed 3 coats of 2k High build primer, mixed 4:1 with fast hardener and 10% 2k thinners. First coat light coat, followed by two wet coats with 7min flash-off time between coats. Used a 1.8mm tip using a good quality taiwanese LVLP gun (Pro-Win).

Allowed 18hrs to cure, then proceeded to sand it down with 500grit (I should have gone coarser and finished at 500, but thats a different issue.

Used a tack rag and spray gun to blow dust into tack rag. Layed down solvent base coat (jet black), also from a local autobody paint supplier. Thinned it down 2:1 using 2K/Universal thinners. Was coming out really thick and gloopy, and turns out the paint supplier has just told me lots of people thin this stuff to 1:1, which would have definitely resolved this.

Thinking the primer might not have been dry enough because it only dried for 18hrs at low temps? It sanded great and didn't clog the paper, so I didn't believe it to be wet?

Primer is KAPCI brand (Made in Egypt), and the solvent basecoat is MIPA brand - both good brands and compatibility should not be an issue as my paint supplier supplies this combo to many sprayers on a daily basis - also sprayed this combo before without issue.

Some pics attached.

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PostPosted: Thu Feb 14, 2019 10:43 am
At 60 degrees, 7 minute flash times seem awful short to me. Regardless of how warm your materials are, the metal is still very cold.
You may be trapping some solvent in the 2k high build. Then when applying your base coat the solvent softens the top of the 2k high build releasing the solvent trapped below.
1968 Coronet R/T


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PostPosted: Thu Feb 14, 2019 10:59 am
'68 Coronet R/T wrote:At 60 degrees, 7 minute flash times seem awful short to me. Regardless of how warm your materials are, the metal is still very cold.
You may be trapping some solvent in the 2k high build. Then when applying your base coat the solvent softens the top of the 2k high build releasing the solvent trapped below.


I like this, its making a lot of sense to me.
Should I give the primer another few days to cure properly? Or will trapped solvents in the coats below be a problem indefinitely?



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PostPosted: Thu Feb 14, 2019 1:19 pm
18 hours cure time is plenty for the primer. I would skip the light first coat of primer and do 2-3 med wet coat or full wet coats. the light and maybe dry first coat could be your problem with the texture in your final finish. also is the paint temp really 95*f ?that's warm, i'm not sure what that does when sprayed onto cool material 60*. oh and nice breakdown of what your doing!
Jay D.
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PostPosted: Thu Feb 14, 2019 4:51 pm
Yeah, just to add on that temp thing.... it is always going to be a better shoot if room temp.s, metal/fiberglass surface temps. and the coatings are within the same reasonable temperature range. When you heat any paint/finish coating up to 95 degrees you are significantly changing the way it will perform. You are causing several of the slower tail solvents to just flash right out of the mix. Missing solvents can make coatings do all kind of strange things.... Oh, and "KITCHEN PAPER TOWELS"..... I'm telling you if they are white in color they have residual bleach in them. Paper towels are made from turning wood into pulp by soaking in caustic lye (sodium hydroxide), then washed, the paper towel material is then brown in color (you've seen brown napkins and shopping bags right?). To make it pretty/clean for the kitchen they soak the stuff in bleach..... Invest in some good shop wipes.....
Metal, wood, fiberglass, we work it all... www.furniturephysicians.com We can restore the irreplaceable!



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PostPosted: Fri Feb 15, 2019 12:17 pm
Alright, thanks for all the ideas.
I've got mutton cloth which I always used to use and will try that again. Dont like it as much as it leaves fibres, but the tack rag takes care of that.

I've blocked down with 320, using dry guide coat, and its super smooth and no micro pinholes/cracks. I think the problem I've been having with the primer is simply just not cutting past the lowest points of the orange peel. I need to sand more to get it flat and use the 320grit, seems to do the trick.

Now I'm chasing my tail a bit as I keep getting cut throughs on the edges. I'm about to lay some more 2k high build on all the parts, but plan is to put 2 coats just on the edges where the cut throughs occur and then a final coat covering the entire piece. That why I should have enough primer on the edges and avoid cut throughs if careful when sanding. I'll keep the DA well away from the edges and do those by hand.

Hopefully the funny paint reactions won't happen when I come to lay the base coat!

*UPDATE*
Scrap that idea, it was bad! The areas not covered directly with the primed had lots of overspray dust on it. Not something I'd do again! Anyway, I carried on just priming the edges, 4 coats and theres enough primer on the flat areas from the first 3 coats, so I'll just sand it all down with 320 again, and hope for no cut throughs.



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PostPosted: Fri Feb 15, 2019 1:10 pm
the idea of this is to sand the part smooth to start with, usually 180 works good. you want to sand all the deep scratches and other defects out or if needed filled. you want it as smooth as you reasonably can get it, then spray your 2k hi build. you also should spray at least 2 good wet coats, if the substrate is rough then 3 coats with good flash times is ok. now you have something to block sand. putting the 2k on with one coat just causes problems like you had and more primer. you MUST make sure your rags are sterol, micro fiber rags work good they are cheep and you can buy them almost anywhere.
Jay D.
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PostPosted: Wed Feb 20, 2019 11:03 am
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PostPosted: Wed Feb 20, 2019 11:20 am
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Ok, so I've finally done the spraying on these parts.

After adding a further two coats of high build and sanding with 320 then 500 using dry guide coat I got a nice smooth finish.

I cleaned with wax grease remover using a brand new microfibre. Still had issue laying the black base coat, first two coats seems to be all mottled and looked to me silicon contamination, with hundreds of microbubbles again - but I'm not sure - would really love to know why I was getting this weird reaction so I can not have this problem in the future. Anyways, I pressed on, and layed a few more coats of black, but layed them in light mist coats and this seems to have kept the base nice and consistant and seemed the more I added the more nice and consistent it layed down. If I ever have problems with painting its always with the solvent basecoat!

Layed down 3 coats of HS 2k Clear and came out lovely. A few bits of trash landed in the surface but I'm used to that as I dont have a booth. Denibbed, wet sanded, compounded and polished the one part so far and very happy with the result.

thanks for all your help and suggestions.
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