Epoxy over/together-with alkyd paint

[timothylltk]

Dear All,

Good afternoon.

We hope this finds you well, well-rested, in good spirit and having a good day.

Can anyone help to find out, whether it is okay, to combine areas painted/shaped with epoxy with areas of existing coating of alkyd paint, please?

We have done repairs above the waterline, on a wooden mahogany hull, that has to our understanding been first primed, and then painted with a long alkyd base paint.

We have taken some areas of the paint off, to fill cracks and to do wood-repairs, and used epoxy in those repairs.

So now there are some areas, that we have repaired and painted/shaped with epoxy, and we are looking at how to finish the painting-job.

Someone told us, that epoxy would crack existing oil-based painting, but we have so far not been able to verify if and how that would happen.

It feels like a waste to take off all the rest of the alkyd paint off, and so far we have been wondering, if it is enough just to paint on top of the epoxied and the existing alkyd-painted areas?

Your help and suggestions and any information are very welcome.

Thank-you.

P.S. Recommendations on paint are also welcome, and I forgot to ask in original posting (edited now), whether it would work, to continue using a similar, or the same, alkyd/oil paint, on top of both the existing paint, and the epoxied areas (with primer/UV protection first for the epoxy?).

 

[Pasarell]

As a general rule you cannot put an epoxy over an alkyd. There are, however, exceptions for some surface tolerant epoxies. If you're done it and got away with it then you may be OK but I would not recommend doing this in the normal way. Normally an alkyd will blister and crack when an epoxy is put on top. If this has happened and you cannot see it because of the epoxy you will have destroyed the adhesion of the alkyd primer and water will get into the mahogany.
It sounds as though your epoxy is a filler. This will be very hard and should be suitable for use underwater but it will not tolerate much movement if the wood is designed to move. I would recommend a polyester filler for use above the water line and I assume that is where you are working since you have an alkyd paint. They are generally compatible with alkyds and more flexible than epoxies. It is best to put the filler over a primer rather than a finish.
You can paint an alkyd over a cured epoxy but need to get a good mechanical key as there will be no chemical bond between the two.

 

[timothylltk]

Thank you for your reply and for helping to make sense of it.

We did not plan to use epoxy on top of alkyd, though, and were concerned whether an area sanded to bare wood, and cracks filled with epoxy, and shaped with epoxy, would somehow cause cracks in surrounding areas, which have alkyd paint as described.

It did not make sense, and we were at a loss what did the people mean who said that epoxy cracks oil-based paints. It makes perfect sense, that it will do that, if used on top of an oil paint.

Based on that, I understand that we should be okay, by making sure we do not have epoxy on top of the alkyd paint, and prepare both the alkyd paint and epoxied areas to receive paint and/or primers (for epoxy/UV protection), and to continue to use alkyd paint on top of it all.

 

[Lakesailor]

Yes. It is absolutely OK to paint over epoxy. Use a 120 grade paper on the epoxy to key it and use a primer from the paint system you are using.

Repaired transom with epoxy filler and coating

Repaired bow and stem

After Painting and varnish

 

[oldsaltoz]

Quote:
<<<It sounds as though your epoxy is a filler. This will be very hard and should be suitable for use underwater but it will not tolerate much movement if the wood is designed to move. I would recommend a polyester filler for use above the water line. They are generally compatible with alkyds and more flexible than epoxies.>>>

Polyester resins will not bond to timber in the long term, but epoxy will.
Polyester can be used in situations like full encapsulation of timber like ribs in hull frame, however the layup should be such that when the bond between timber and cover breaks the layup is thick enough to provide the required stiffness. This is why foam replaced timber in hull stiffeners /ribs /frames etc, cheaper and lighter as well as being easier to shape.

Avagoodweekend......

 

[rjp]

I have been in exactly this situation and have found no problems painting over epoxy filler with a conventional alkyd paint - still fine after 3 years and you would never know where the filled area was on the hull. I've been less successful with Blakes deck paint over epoxy though where I've found adhesion to be poor. I think this is due to the paint rather than the epoxy though. Never use polyester resin on wood - it won't adhere long term.

John