Car Bog on Epoxy

[surfynic]

Hi all!
We have been doing a MAJOR re fit on our Simpson 40 catamaran and have just spent the last year fairing the decks, at the start we had a painter slash supposed boat builder working with us and he was showing us how to fair the boat. To do this he was using car bog (goldrush) as it went off fast and was good to sand. Now thou we are pretty well finished and had someone come and say that the car bog will crack so as you can imagine we are a bit panicky...

Does anyone have experience with a situation like this? The boat is a ply with epoxy glass over the top construction. We are considering laying a surface tissue with epoxy over the whole lot and faring that back with q cells to enclose the bog... We are open to suggestions!!!

Thanks
Nic

[JPA Cate]

A couple of caveats: my boat is western red cedar strip plank with epoxy and glass over alll, and some tiny bits of fairing have been done with red car filler. They are so thin that they have never given trouble.....but the boat is very fair. YMMV.

[nimblemotors]

I assume by 'car bog' you mean the equivalent brand name 'bondo', which is essentially polyester resin and talc powder. Having done a lot of body repair in my day, it will crack if put on too thick in one layer, 1/4in is too thick. The best stuff has fiberglass 'hair' in it, so it can be put on thicker and won't crack as the glass holds it together.

Regular bondo will absorb water, so if used on a boat, you must coat it with something waterproof. Epoxy over the whole thing with veil sounds like a good idea. Get ready for even more fairing...
Perhaps just painting with epoxy barrier coating is enough?

And not to pick on you, but another case that fairing an old boat is a lot more time and effort than building a new boat from a female mold..

[surfynic]

Hi thanks for your responses. It has been put on it lots of very thin layers, and built up to a maximum of 3-4mm but mostly a bit thinner than that. It's never been built up to 4mm in one go thou, not sure if that makes any difference?

[Parmenter]

I'd be worried with that sort of thickness.
Like Ann T. Cate, I have some in mine, but only for filling divots and pinholes that weren't filled at first high build/shadowcoat stage.

Originally Posted by nimblemotors

The best stuff has fiberglass 'hair' in it

It'd be a bitch to sand and its still poly, so not really suitable for an epoxy build.

[surfynic]

Maybe 4mm is an exhaduration, maybe more like 2mm..

[Parmenter]

Originally Posted by surfynic

Maybe 4mm is an exhaduration, maybe more like 2mm..

Over an area the size of the palm of my hand maybe, any more than that I'd be concerned.

[surfynic]

Hmm well I'd say I'm concerned..

[Redreuben]

The bottom line is car bog is polyester and doesn't like bonding to epoxy unlike the reverse.
If it had lots of raw sanded glass to adhere too it may be okay but are you willing to put the topcoat on and risk it ?
If it was mine I would replace it with an epoxy based filler.
Then you can finish the job with confidence.

[JPA Cate]

Originally Posted by Redreuben

The bottom line is car bog is polyester and doesn't like bonding to epoxy unlike the reverse.
If it had lots of raw sanded glass to adhere too it may be okay but are you willing to put the topcoat on and risk it ?
If it was mine I would replace it with an epoxy based filler.
Then you can finish the job with confidence.

^^^

And yet, Red Rueben, the chap who built out boat used it, and so far ~22 yrs., it has been okay, the red car filler available in Australia, on our wrc strip planked boat, with expoxy in and out, so a very particular situation, may not extend to other boats...

[surfynic]

Originally Posted by Ann T. Cate

^^^



And yet, Red Rueben, the chap who built out boat used it, and so far ~22 yrs., it has been okay, the red car filler available in Australia, on our wrc strip planked boat, with expoxy in and out, so a very particular situation, may not extend to other boats...

But there's not much on yours is there? 22 years is pretty good! We've put it straight on glass that we've sanded rough then cleaned with thinners just before application of the bog..

I'm thinking I might go down to
Tomorrow and try a few strength tests and drill a few pin holes to check the thickness of the bog and then see about how much I could sand off, if not all of it... Almost a years work down the drain!

[boatsmith]

Polyester based fillers and putties should not be applied over epoxy. Much to frequently it will lead to problems downstream. The problems I have seen is the epoxy leaching though the poly and ruining a finish and a glueline failure where the filler will sheet off in places.
Once we are in the paint process (AwlGrip) we have used bondo for small faring details that were missed. I was nervous when my painters showed me this . They said this was SOP. I checked with my suppliers and the AG tech who said that even though their printed material says not to use it , that that was ok.
On a wood epoxy boat as primary fairing I would be VERY uncomfortable!!

[Redreuben]

Surfynic,
I have worked in boats and fibreglass for years, polyester over epoxy is just not done.
However truth is stranger than fiction and people do with all sorts of unconventional things with materials and get away with it.
Sheathing ply with polyester is frowned upon and poo pooed now, but millions of boats have been built that way.
Is there a local marine spray painter you could get an opinion from ?

[surfynic]

Yeah it would be good if we had a good marine painter but unfortunatly that's the problem where we live in that we don't have anyone good around here and constantly get differing opinions!

[Redreuben]

Talk to Gary Martin at Boat Paint in Freo.
Boat Paint Fremantle, Perth, Marine Paint WA, Retail & Trade Boat Paint, Master Marine Painter, Private & Commercial Boat Painting, Boat Painting Supplies WA, Commercial Boat Painting Perth, Sealants, Hull Cleaners, Polishes and Compounds, Fibreglass