West System 410 under polyester type top coat

So I am restoring a 18 foot runabout, it is made of gelcoat and fiberglass...I've been using epoxy to repair various areas of the boat on the topside and transom...I am practicing fairing the areas before putting a top coat on and I found epoxy with a mixture of fumed silica and 410 works well because the silica stops the running and the 410 makes it easier to sand. Just noticed on the 410 bottle today you must use an epoxy barrier coat before adding gelcoat, polyester paint, etc. Is that accurate? Why is that?

I worked on smaller projects and rolled Duratec over the 410 without any issues I could tell.

I am thinking I want to have the topside of the boat sprayed with gelcoat, it's quite a big area to roll and it will easy to repair in the future rather than having to grind away old paint if I need to fill a hole or something. Let me know what I am missing... I can post some pictures of my boat if it would help.

 

410 or other micro-baloons look best if heavily primed. Sanding breaks some of the balloons leaving a slightly porous surface. The opened balloons should be filled with something prior to painting. If not the fairing patch often appears to be a lower sheen than the rest of the surface. Any quality primer will do.

 

Forget about gelcoat completely, complete all repairs with the epoxy bog, Blueknarr very rightly pointed out that hollow sphere fillers need a good primer with high solids content so you don't get that porous substrate showing through the gloss finish, a good 2-part Polyurethane finish is virtually a job for life, far better than gelcoat.

 

Gotcha so it's more about the surface texture itself than if a polyester top coat like gelcoat wont cure and stick?

I plan to continue using epoxy for the rest of the build, what primer should I use over the West System 410 fairing filler? I was thinking a Duratec primer because it will be easy to sand.

I do not know much about the 2-part polyUrethane top coat options. Is this something that can be sprayed on? And does epoxy stick to it if you have to make a repair after applying the top coat?
What about below the waterline on the transoms and sides of the boat, what should I be using for those areas?

 

Durateck would be an awesome choice.

Two part polyurethane is best sprayed. Cheapest from an automotive paint supplier (must be sprayed). There are marine two part polyurethane that can be rolled and tipped. But it takes many years of experience to produce excellent results. So still better sprayed.
Polyurethane is good for the top, sides and bottom of trailered boats. Vessels left in the water full-time need special anti-fouling paint.
All repairs should start at the substrate and never be on top of paint or gelcoat.

 

I would not be depending on polyester to "stick" to much at all, it isn't adhesive to too many things, including epoxies. Yes, 2-part PU, which is often used with an epoxy primer base coat, can be sprayed, but is toxic, and it is better to roll on and "tip" with a good quality brush, from the health perspective. It can be coated again with epoxy down the track, if need be. 2-pack PU is suitable for underwater areas or anywhere really.

 

Can you guys give a name of a brand for an epoxy primer base coat and 2 part polyurethane? Would one be Alwgrip?

As far as epoxy repairs down the road. I feel putting epoxy onto sanded gelcoat is easy to repair because the gelcoats sands finely. If I have to repair an area or fill a hole in the fiberglass, how easy would it be to sand the 2 part polyurethane? Would it gum up the sand paper? Im just trying to get a sense of how different the polyurethane is from gelcoat because I have always thought boats should be made with gelcoat for durability. I think I read somewhere that the 2 part polyurethanes are more UV resistant than gelcoat?

 

The Duratec products work great with epoxy, the problem is mixing epoxy with micro spheres, that’s what creates bonding problems for Duratec primers.

A layer of neat epoxy over the spheres solves the bonding problem.

It doesn’t matter at this point, but there was no need for epoxy on this project, it sort of complicates the process and increases the cost.

You don’t need any special paint for a boat that is stored on a trailer, most any automotive paint with a hardener will work. Many of these paints are easy to use and repair if needed.

Gel coat can be used, it does take more time to do though. Depending on the exact paint, it may or may not be more durable. Most paints do weather better than gel coat.

 

Yes

 

Cool thanks for the help guys I think I am getting it, just trying to finish the last of this boat before I take it to a guy to get it painted and wanted to know a little about what to expect

 

How is it to sand an epoxy primer? Does it gum up or is it like a polyester based primer that finely sands. I know epoxy can sand to a fine dust eventually but I've only noticed that after several weeks of it curing...

 

Don't confuse epoxy resin and epoxy primer.
Primers have lots of solids which make sanding a breeze.
Just give it a day to cure. They are designed to be sanded.

 

I used 2-part PU after epoxy primer on a glass boat 30 years ago, it has been out in the weather in a sub-tropical climate for decades, and is still glossy ! Way better than gel coat

 

I used an architectural 2part PU that had a 50 year warranty!

Gelcoat is a dinosaur