I have a hard top for an MX5 and I think it may have signs of osmosis. I need to respay it and was wondering how to prepare it.
I gave it a touch up last year and its shed its paint already. Any tips would be helpful thanks.

I suspect you'd need to bake it in an autoclave for a fairly long time (days) - that's how we have to dry out panels that have only been in ambient air before coating/bonding etc.

It depends on the size, but the principal is:

- grind out the blister removing the affected material (it will smell like vinegar)
- let the area dry out thoroughly. This can take weeks if the blister is large - you can seal over the area with plastic to check the smell periodically. The vinegar smell will disappear when it is fully dry.
- once dry, fill with epoxy resin/paste if small. If larger, rebuild the laminate and finish with gelcoat/paint as needed.

Thanks for the advice, I will sand down and leave to dry out. Hopefully the weather might improve soon.

In large boats the treatment is to sandblast off the gelcoat and re-gelcoat the hull. Brothers 41 foot Morgan sailboat had the problem.

Osmosis in grp panels isn't as simple as it seems.


Gelcoats, particularly polyester based ones, allow water through. Epoxy based ones are much more resistant to water flow but these are usually not used, certainly in cars, due to cost.

Once water wicks through the gelcoat then it reacts, particularly if the panel wasn't cured properly in the right temperature and humidity conditions, with the resin forming a range of acidic chemicals, hence the smell and taste. This accelerates any water absorption by forming a strong solution that does provide an osmotic effect.

Just rubbing the blisters down and repainting will not provide a long term solution as the process will quickly re-establish.

Drying will be better but won't remove the acid compounds from the substrate. Epoxy coating will only stick if the moisture content is low enough.

Nowadays the best cures for boat hulls are seen to be stripping off the gelcoat, hot steam applied under pressure and vacuumed out, removing as much soluble problems as possible. Then drying, followed by an epoxy coating.

Not much of this is practical for a grp hardtop but it shows that if you have been getting blisters then rubbing down and repainting will be a short term fix. As a minimum, strip any gelcoat, hot water washing and then drying out before applying an epoxy layer would be a medium term solution.

Another method is to vacuum bag and bake with an absorbent, porous material in the bag to soak up any moisture.

Lots of trouble for a hard top I know.

Maybe a ghetto solution would be to use some bin liners to make a bag. Wrap in some towels, add silica gel pouches, seal it up and leave it outside on a sunny day. No vacuum needed.

Just an idea...