Almost ready for topcoat. Is this right??

Discussion in 'Materials' started by snowbirder, Apr 18, 2015.

  1. snowbirder

    snowbirder Previous Member

    We are sanding the high build primer with 320.

    Everything I've read said this is a good grit to use before topcoat.. but!!!!

    The sides of the hull now look like gelcoat with some dust on them! Very shiny. Slick. Smoith. Reflective.

    That doesn't seem right in my mind. Seems like it shouldn't be smooth or the topcoat won't stick.

    Does it seem right? I'll go take a picture...
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    It should have a sheen if the sanding took all the previous marks off.
     
  3. snowbirder

    snowbirder Previous Member

    Pictures... it's smooth like freshly waxed gel coat.


    [​IMG]


    [​IMG]
     

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  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Some primers dry with a modest sheen, but once sanded dull out to the flat you're use to. 320 is fine and as high as I would take a primer, that's getting a roll and tip topcoat. Most hand applied topcoats will cover 220 grit with two coats. You might want to try backing down to 280 or 220 and having a look to see if the dullness returns? It may just be a function of the primer, but yeah, I'm like you and would question it, so perform this simple test (use a coarser grit) just to check the sheen quality. Personally, I rarely find the need to go over 220, as topcoats, once bulked up to proper film thickness, will fill these scratches. If the brand of paint is especially thin, I might use 280 and very rarely 320. These are "micro" grits and the very low end of the polishing process. Unless spraying a custom paint job (read many thousands of bucks), where you'll refine the coatings through 3,000 grit, before all is said and done, 220 is all you need IMO.
     
  5. snowbirder

    snowbirder Previous Member

    I have sprayed the whole thing (all high build was sprayed) and yes... it is a many thousands of $$$$ custom paint job. Plan is to invest in the exterior, then finish interior so it's ready for weather. I plan to spray the topcoat in a quick misting pass, then within half an hour, a full depth of thickness pass.

    This primer is a 2 part lpu high build in gray.

    The topcoat is Nason ful-thane custom tint. Very expensive stuff, though less than marine urethane.

    This is the final paint for the boat, so I don't want to screw it up.

    The sheen in the pictures is after 320 sanding. The texture after spraying the primer is pretty flat. It got polished it seems from 320 which has me nervous.

    The 320 was run on a pair of little round DAs running off the compressor and it was dry sanding.
     
  6. snowbirder

    snowbirder Previous Member

    PAR- You're invited to the launch party in a couple months, by the way. ;)
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Cool on the party, I'm always up for some fun . . .

    Again, try a coarser grit and see what you get. You can always refine the top coat if you need to (bulk it up and knock it flat). The real goal is good tooth. Assuming the surface is fair and relatively smooth, shoot the top coat and polish it smoother if necessary, knowing it's got a good grip on the primer.
     
  8. freddyj
    Joined: Oct 2013
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    freddyj Senior Member

    320 is very fine on a da sander. I am an automotive painter and I can usually finish with 220 0n a da, or 400 hand sanding. This is for a single stage paint. Basecoat/clearcoat, I go one step finer.
     
  9. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    Auto Painters like freedy do this stuff every day.
    I'd be doing a prelimnary test shot in the tunnel somewhere, then lay on some masking over some razor cross cuts, peel & see how she sticks.
    Saw a pretty massive paint muck up on a 40' cat once, pretty sure they went to 500, at that stage the fine particulates might be acting as polish, cant remember the brand but on doing cove line masking lost a few strips of paint....
    I know altex warns also at least with their elite range max grit 240 & also to not use stearated dry lubricated paper- I think most machine paper is, wet & dry is not apparently, some paints will stick through some gloss no problem, I suppose by some solvent aggression or are just really sticky?- it's a whole chemical science that I leave for the formulators....

    Jeff.
     
  10. snowbirder

    snowbirder Previous Member

    Whew. Thanks, everyone. Glad I asked. We'll scuff it up with more coarse grits now and do the rest of the boat with more coarse stuff too.

    This is single stage. It's just high build primer in the pictures.
     
  11. snowbirder

    snowbirder Previous Member

    FreddyJ- Any suggestions on a good gun I can spray upside down with (cup not attached)?

    Was using a small pressure pot made by Grizzly.

    This one:

    http://www.amazon.com/Grizzly-H7668-Professional-Spray/sim/B000E35ZOE/2

    As of today, both are broken. Seems the air regulator went on the second one and I have no idea what happened to the first. They are clean, but don't siphon the paint up well from the pot.

    So what's a good spray system where I can hold the gun upside down, keeping in mind I have to throw it away in a few weeks?

    These Grizzly POSs are going to ruin my topcoat. When they screw up on primer, not an issue.
     
  12. snowbirder

    snowbirder Previous Member

    Looks like I can pick up Eastwood and Devilbiss guns for around $200.

    Seems the way to go. But!!

    How do I spray a ceiling with a gravity cup gun???
     
  13. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    A lot of Polyurethanes 2k and one pack give you a three day window for chemical bonding. So no rubbing down is required in that period.....;) so maybe the sheen is completely irrelevant, right now I'm varnishing one boat (post 2 layers of epoxy ugh!) and that (varnish) has been cut with 320 and has a very light sheen, but subsequent bond is no problem.

    Like PAR and others, I'd probably use 240/280 on a high build polyester or epoxy primer and cut it wet whilst in the 'soft' window. 320 is good for the top coats themselves and even 400 for varnishes 2k marine and other high quality stuff like automotive, aircraft paints etc.
     
  14. freddyj
    Joined: Oct 2013
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    freddyj Senior Member

    You don't really need a pressure pot. A gravity feed gun with 3M pps cup and liner will spray upside down fine and lay our the paint real nice. Unless you are very experienced, a pressure pot will usually put down too much material.
     

  15. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Some folks like the gravity feeds, but I'm back to my pressure pot and siphon, because of the weight and versatility. Crawling around in a tight bilge, engine room, prop tunnel or something with a gravity gun, is just asking for a headache. They're great if you have good access.

    Don't trust the 48 - 72 hours chemical activity stuff the manufactures suggest are possible. They don't have to repaint the job you trusted to their recommendations. A good painter will know about how long he can "stretch it", before needing to scuff it up. This is an area you want to be conservative. If you're working a paint that can be color sanded relatively quickly, it's vehicle has flashed enough that you don't want to trust a large "active" window. If on the other hand, the paint needs a fair bit of time to get hard enough for a color sand, it's probable you can overcoat in this window.
     
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