Gelcoat / osmosis / paint on GRP question(s)

[snappie]

Hi guys,

Working on stripping back the paint on my snapdragon today.

Working inside the cabin, and on the cabin roof /sides (above the waterline).
For the snapdradgon, it looks like the sides / roof were moulded separately and connected to the hull moulding.

My question is as follows:

It looks like there is a lot of little osmosis blisters (maybe an eighth of a centimetre, on avg) but there are a lot of them.

The PO had sprayed on a heavy non-slip paint, over a thick white undercoat on the cabin sides / roof. I didn't see the blisters below this. When I'm sanding this back (Random Orbit sander, 40 grit), I can see where the sander is smoothing these out.

Is this a problem? Do I need do something else than sand back to a smooth gelcoat, and then paint (for the first time in a few years).

Also, on the hull moulding, it looks like there is just paint over the fibreglass. I'm a bit nervous sanding this back with the random orbit, as it don't want to damage the glass. its the same awful mixture of non slip paint and the white undercoat (albeit not as thick).

 

[rob2]

We may have some little misunderstandings her in nomenclature for the various part of the boat - not surprising as some terms seem to have differing connotations according to context! I haven't been up close to a Snappie for a few years, but remember them as being of fairly standard construction The hull moulding has the keel(s) bolted on and runs up to the gunwhale. The hull above the waterline up to the gunwhale is usually referred to as the "topsides" - the area which is moulded or painted in the chosen colour for the boat. A second moulding is made comprising the deck and coachroof (which includes the cabin sides). Basically the topsides would gain no benefit from having non-slip paint and the finish needs to offer adequate water proofing and decoration. Below the waterline is the most uysyal place to find osmosis as the continuous immersion allows water to permeate very slowly into the fabric of the hull where it reacts with any uncured elements of the layup. Bursting any blisters which are osmotic will release small quantities of a vinegary smelling liquid. They can be treated by thoroughly sluicing them out to remove any remaining uncured elements, drying and skimming over with epoxy. Epoxy has far lower permeability than the polyester resin used in the original layup and also bonds better to the existing, fully cured GRP.

Above the waterline, whether the topsides or the deck moulding the risk of osmosis is much less as it is not permanently immersed. It is not that uncommon however to find air bubbles and voids in the layup which acan simply be made good and painted over - again epoxy does a better job of adhering and can then be sanded flat and painter over. Next time you find another blister, prick it open and have a sniff for vinegar and scrape at it carefully to find whether it is in the fibreglass or the paint.

Lastly, don't hesitate to come back with further questions as you discover what is going on. A few pictures of what you're finding will always help others to advise on what treatments are appropriate.

Rob.

 

[snappie]

We may have some little misunderstandings her in nomenclature for the various part of the boat - not surprising as some terms seem to have differing connotations according to context! I haven't been up close to a Snappie for a few years, but remember them as being of fairly standard construction The hull moulding has the keel(s) bolted on and runs up to the gunwhale. The hull above the waterline up to the gunwhale is usually referred to as the "topsides" - the area which is moulded or painted in the chosen colour for the boat. A second moulding is made comprising the deck and coachroof (which includes the cabin sides). Basically the topsides would gain no benefit from having non-slip paint and the finish needs to offer adequate water proofing and decoration. Below the waterline is the most uysyal place to find osmosis as the continuous immersion allows water to permeate very slowly into the fabric of the hull where it reacts with any uncured elements of the layup. Bursting any blisters which are osmotic will release small quantities of a vinegary smelling liquid. They can be treated by thoroughly sluicing them out to remove any remaining uncured elements, drying and skimming over with epoxy. Epoxy has far lower permeability than the polyester resin used in the original layup and also bonds better to the existing, fully cured GRP.

Above the waterline, whether the topsides or the deck moulding the risk of osmosis is much less as it is not permanently immersed. It is not that uncommon however to find air bubbles and voids in the layup which acan simply be made good and painted over - again epoxy does a better job of adhering and can then be sanded flat and painter over. Next time you find another blister, prick it open and have a sniff for vinegar and scrape at it carefully to find whether it is in the fibreglass or the paint.

Lastly, don't hesitate to come back with further questions as you discover what is going on. A few pictures of what you're finding will always help others to advise on what treatments are appropriate.

Rob.

Cheers for that - I will do. I'll post up some photos.

This would be the coachroof that I'm referring to. Can't see any blistering below the waterline or on the topside (outside). No idead why the PO sprayed on this paint inside the cabin, maybe to hide the blisters? Proving a pain to get out.

The only blistering seems to be inside the cabin - reading this site (http://mike-menzies-marine.co.nz/new_page_3.htm) it seems to fall into the gelcoat blister category.

 

[snappie]

Photos:

The area I'm working on at the moment

There are a couple of blisters here, I'm not getting a smell of vinegar but the blisters are small, the fluid coming out has a tiny volume, I've a bad sense of smell and the cabin doesn't smell great anyway (damp and oil).

You can see the effect before its sanded back fully:

Close up of the last pic

I'm assuming that I just keep sanding until it is smooth, then clean it down and then paint up?

 

[rob2]

It would be most unusual to have osmosis only inside a cabin! When a moulding is laid up, the gelcoat is applied fairly thinly - old boats have been known to be loved to death, polished annually until they go through the gelcoat. Obviously only one side of the moulding has gelcoat when it comes out of the mould - the side which was in contact with the mould, usually the glossy exterior. Finishing the interior surfaces can be done in several ways. Initially it will have an uneven texture with the glass matt just below the surface. Some manufacturers covered it in panels, others glued a lining material to it, but it sounds like your Snapdragon had flowcoat painted over it. Flowcoat is the same material as gelcoat, but includes wax in the mix which rises to the surfaceduring the cure. The wax precludes air which would otherwise prevent full curing. It is painted on fairly thickly, sometimes in several coats to achieve an even colour (washing the wax off between coats). Being quite viscous, it is not unusual for the finish to have some air bubbles and voids - but they are not osmotic, there is no water permeating through the surface. If all that matches what you are seeing, then rub it flat and paint it - job done!

Rob.

P.S. All above written whilst you were posting the pics. I'd just rub down, make good if required and paint.

 

[snappie]

It would be most unusual to have osmosis only inside a cabin! When a moulding is laid up, the gelcoat is applied fairly thinly - old boats have been known to be loved to death, polished annually until they go through the gelcoat. Obviously only one side of the moulding has gelcoat when it comes out of the mould - the side which was in contact with the mould, usually the glossy exterior. Finishing the interior surfaces can be done in several ways. Initially it will have an uneven texture with the glass matt just below the surface. Some manufacturers covered it in panels, others glued a lining material to it, but it sounds like your Snapdragon had flowcoat painted over it. Flowcoat is the same material as gelcoat, but includes wax in the mix which rises to the surfaceduring the cure. The wax precludes air which would otherwise prevent full curing. It is painted on fairly thickly, sometimes in several coats to achieve an even colour (washing the wax off between coats). Being quite viscous, it is not unusual for the finish to have some air bubbles and voids - but they are not osmotic, there is no water permeating through the surface. If all that matches what you are seeing, then rub it flat and paint it - job done!

Rob.

P.S. All above written whilst you were posting the pics. I'd just rub down, make good if required and paint.

Cheers for that - there is some liquid coming out of the bubbles, but they seem to exist entirely within the gelcoat (flowcoat?), Im not seeing the glass behind them seeing warps and when I sand back to a smooth finish, Im still well within the gelcoat.

I'll just keep sanding until the pockmarked vista has gone, then will clean and paint over.

In relation to my second question, below the coachroof (the topside - is it still called that when referring to the interior?), it seems there is just glass and no gelcoat, so the paint has been applied to this.

Is the random orbit sander ok to run over this?

Thanks!!