Priming and painting Epoxy

Discussion in 'Materials' started by mvboatbuilder, Jan 14, 2012.

  1. pauloman
    Joined: Jun 2010
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    Location: New Hampshire

    pauloman Epoxy Vendor

    only epoxies rated for high temp need a post cure. Essentially they are a blend of ambient curing agent and a high temp curing agent that needs elevated temps to cure. using the regular curing agent in the blend allows the resin to become dry to touch, but leaves the high temp curing agent largely un-crosslinked until heated in elevated temps.

    note that pretty much all epoxies take about a week for full cure. you would never perform lab tests on any epoxy sample not cured for 10 days or longer

    paul oman
    progressive epoxy polymers inc
     
  2. CatBuilder

    CatBuilder Previous Member

    Interesting, Paul.

    So do you mean epoxies rated for 120F or more, or is 120F considered high temp? I mean, the surface of a boat in the tropical sun can push those temps sometimes, right?

    I had always wondered if there was also a time component to this stuff.

    As in many epoxy distributors end up thinking you are making a wind turbine blade in the morning, then trying to spin it up on a tower later that afternoon. :)

    Building boats with less labor than ideal, parts sit for months in 80-100F temps. I notice weeks later, you can't even flex them, even with the infusion epoxy that they suggest to post cure.

    As time goes by and hot day after hot day take place, the stuff gets very stiff and hard.

    Every component of my boat will sit for at least a month at 100F daily average temps at the end of this build (July in the South). I keep wondering if this is enough. Lots of people say no, but even the infusion epoxy manufacturer said it wasn't absolutely necessary to post cure in this climate, even though it was still recommended.

    Confusing stuff.
     
  3. pauloman
    Joined: Jun 2010
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    pauloman Epoxy Vendor

    the Tg (where epoxies begin to soften) varies a bit with different epoxies. Usually about 160F in dry conditions and about 140F in wet conditions.

    special high temp epoxy curing agents don't ever set up in temps under about 100-125 degrees, so they are mixed with 'regular' curing agents, but then need that high temp post initial cure to get the high temp curing agent to set up.

    100 plus temps will greatly speed up the cure of regular epoxies which in normal temps take about a week or 10 days for 100% cure. - added solvents and fillers (maybe) can extend this cure time.

    progressive epoxy polymers inc.
     
  4. AndrewK
    Joined: Mar 2007
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    AndrewK Senior Member

    This can be missleading, regular epoxies in boat building use curing agents that will give a gel time of 20 -200 minutes. Even the 20 minute epoxy will not be 100% cured after 10 days.

    CatBuilder; the only way to know how much your resin system will benefit from a post cure is to get the data. But as you know this easier said than done.
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Cure rates can be misleading. Most epoxies take about a month to get to 100% cure, but will be 90% - 95% in a week to 10 days. If you're testing your glue lines to 90% of breaking strength, you'd be best advised to wait a month. As to post curing, well it's formulation specific, so contact your vendor for a physical attributes sheet. Most of the time you can just call their tech support line and get all the answers you need.
     
  6. mvboatbuilder
    Joined: Jan 2012
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    mvboatbuilder Junior Member

    Hello OK back at it after a break in the building just rebuilt a Herreshoff 12.5 and small kitchen now back on the power boat . My question is before applying epoxy filler coats do i need to wash the hull or just lighty sand before the next step When i stopped the hull had been glassed with 10 oz cloth and 2 coats of straight epoxy were applied. thanks Tim
     
  7. pauloman
    Joined: Jun 2010
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    pauloman Epoxy Vendor

    being a standard chemical reaction, epoxy crosslinking is (in theory) doubled or halved with every 10 degree C (18 F) change in temp.

    regarding mvboatbuilder - always best to wash off dust/possible blush and sand a bit for a profile. That said most folks, especially if using non blushing epoxy, don't bother and generally things turn out fine. (ditto for DIY garage floor epoxies, etc.)

    paul oman
    progressive epoxy polymers inc.
     
  8. mastcolin
    Joined: Jun 2005
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    mastcolin Senior Member

    Don't know if I agree here. Absolutely sand, after a good wash down with detergent solution.

    All epoxies have a very limited overcoating window without sanding. On pigmented systems it can be reasonable depending on supplier. On a laminating layer ie no pigment, you are risking your work. It doesn't take long to sand it to remove(minimise) risk.

    All epoxy curing agent component have a sidereaction that occur on surface. A visible or tangible sticky 'blush" is extreme case, but they all have the sidereaction and it's downside to adhesion to various degree. And it's glass smooth which is never good idea for adhesion.
     
  9. bntii
    Joined: Jun 2006
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    Location: MD

    bntii Senior Member

    You need to thoroughly sand out the entire hull.

    The goal here is to get good key in for the next coats & you will have to see what you have on your surface to tell you how much to sand out the boat.

    As you start to sand you will see all the drips and dust bumps start to flatten first, next you will see the epoxy sitting on the weave of the fabric start to go. The goal is to have the surface largely free of shinny new epoxy. If you lack sufficient fill coats, the sand out will begin to take up some of your fabric.

    For the next build- trowel bog onto the green layup, sand/fill/fair this & step next to fairing primers, 545 then topcoats.

    When you get to prep for topcoats- don't sand through the 545.. If you find your self doing this you are still fairing in this coat and should reapply the 545. You should not shoot the hull on a quilt-work surface of different materials. The only permissible exception is small bits of glazing in pinholes etc.

    Tip- don't brush/roll/mop on 545- buy a cheap HD gun and spray it or buy some good gear and live the good life for your present and future projects.
     
  10. mvboatbuilder
    Joined: Jan 2012
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    mvboatbuilder Junior Member

    Thanks what do you mean by trowel bog? and what is a HD gun? I have a hvlp that i use for cabinets thanks
     
  11. bntii
    Joined: Jun 2006
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    bntii Senior Member

    Bog is lightweight fairing compound- you trowel it on..

    HD is Home Depot.
    I just picked up another primer/varnish gun there for $45 bucks.
    This one:

    http://www.homedepot.com/buy/paint/paint-sprayers/husky/gravity-feed-hvlp-spray-gun-67678.html

    I wouldn't use it for topcoats on a whole hull shoot, but it does great on small work in primers and topcoats. It might do for a smallish hull but I haven't tried it yet.

    My go to gun for larger work is a Binks Mach 1 SL with a 3qt Binks pot.
    I have all the important bits replaced with C.A.Technologies parts and it shoots like a dream.
     
  12. pauloman
    Joined: Jun 2010
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    pauloman Epoxy Vendor

    I want to confirm what PAR said in post #17 about epoxy and alkyd enamels.... PAR is certainly one of the most knowledgeable folks on this site.... (and right about 98% of the time - wink wink!)

    Generally epoxies are sort of a universal primer (especially for 2 part lpu coatings). However in rare conditions of temp/humidity etc. alkyd enamels simply will not harden over epoxies (and I can never repeat this when I want to for additional testing). Latex, spar varnish, mcu - moisture cured urethanes - don't have this unique sometimes problem.

    But a pigmented primer is nice anyway. It shows spots that need additional sanding, filling etc. that are hard to see in a clear epoxy against a multi colored background of old paint, fillers, putty etc. And the primer coat is easier to apply than extra layers of topcoat, and provide a uniform and constant colored, fresh base coat for your topcoat.

    my favorite is an mcu primer which gets darker in color when sanded (thus showing high spots on the surface).

    paul oman
    progressive epoxy polymers, inc.
     
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  13. mydauphin
    Joined: Apr 2007
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    Location: Florida

    mydauphin Senior Member

    FYI, Ideally I would wait 2-4 weeks to sand and paint to let epoxy harden up or fail. Found some epoxies soften or cracked and had to be redone.
     

  14. mvboatbuilder
    Joined: Jan 2012
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    mvboatbuilder Junior Member

    Hi there i am back at the powerboats making a final push to get them done and sold, topside is just finished awlcraft 2000 came out pretty good and i am happy. Next up is the bottom it is made up of glass over okoume ply with multiple skims of epoxy then primed with 545. the boats will be mostly trailered so looking for some suggestions as to bottom painting was looking at VC perfomance epoxy but really don't want the bottom to be white. Thanks in advance
     

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