sealing fairing compound?

My Wa'apa is coming along



That's with the fiberglass freshly on.

And I've already put lots of fairing compound over the fiberglass and I'm not sure if I need to seal the microballoons with epoxy prior to primer. My thinking is that I spent so much money to use epoxy to make a fairing compound that I shouldn't have to go the extra step and seal it--otherwise Bondo would have been just as useable (probably not.)

 

Use an epoxy primer and you may not need to worry.

 

Bondo is a polyester product, won't stick to epoxy.

 

I don't recommend Bondo for any use on a boat, but you can get it to stick well enough for fairing applications, if top coated with epoxy and preferably with a sheathing too.

I recommended all raw fairing materials, get top coated with straight epoxy, once you have this fair and smooth. This seals the pores of the fairing compounds, also any pin holes and other minor defects, plus provides a tough, more importantly uniform base coating. so you can get aggressive when sanding the primer or top coat, if you elect to got straight to top coat, which you can on most paints.

Epoxy primer will provide a good "bond promoter" to the substrate, but really doesn't seal things at all. It just ties the top coat to the substrate. Since most fairing compounds can absorb moisture pretty readily, it should be sealed, which a straight coat (or two) of epoxy will do. So, apply the epoxy over everything and scratch it with 100 - 120 grit before primer. If going straight to top coat, scratch it with 150 - 180 grit and apply sufficient top coat film thickness to fill these tooth marks, while having enough to still smooth and polish.

 

You can use neat epoxy resin to seal but you will struggle to get a smooth surface with a roller. So you will have to fill this structure if you are wanting a smooth finish with...what exactly? More filler? Thick primer??

The sealer epoxy is also very hard and you have a limited overcoating window when you do want to paint it.

Use high build type solvent based epoxy primer. This will flow better meaning less sanding subsequently. Depending on the final finish required and type of primer you can even skip most of the sanding stages.

Make a very runny filler mix as last layer. This will seal most of the pinholes if you scrape it on with a sharp blade. A metal ruler is ideal as has been suggested here if you can't get a real filling blade.

Don't ladle on the primer on the filler mix. It isn't the epoxy that is the problem, it is the microballoons. These absorb water and solvent. Just apply the primer in recommended thicknesses and look out for the temperature being warm enough to allow the solvents to escape before overcoating as per spec. You can physically apply coat on coat to mm thickness but if you do it too fast/too thick you will trap solvents in the bubbles and get all sorts of problems.

 

Thanks for the advice fellas

 

Epoxy primer is what is used in the shipyard. Awlgrip or International . Primer is expensive .

I suspect epoxy resin would build a vapor barrier cheaper than paint.

 

Epoxy can be applied without the stipple a roller leaves behind. A squeegee is the best method (unless you can spray, which most can't), though you can roll and tip too. Epoxy primers aren't completely water proof and I've had issues with them over filler, below the LWL on moored or berthed boats. This is the reason behind my recommendation for a pure epoxy sealing, before any paint. Yes, you do have some smoothing, but this is assumed with any paint job. You'll block down the primer, before top coating, so not much of a problem, unless you where silly enough to apply epoxy with a wire brush or roller. Even in these cases, you could quickly smooth the hull with a DA, before moving onto primer. The factories and most repair yards do a lousy job at fairing and smoothing. If you want this quality, then by all means just top coat the filler with primer and hope for the best. Simply put, I've never seen a fair production boat, though they are usually smooth (big difference). If production boat quality is all that's desired . . .

 

There wouldn't be any need to seal above the waterline would there?