Blister treatment

[Witchwood]

Having knowingly bought a boat this season with osmotic blisters (none bigger that 15mm), I will be looking to treat them individually over the course of the winter and early Spring whist she's out of the water. It is not financially viable to go for the full professional treatment, nor do I think it's necessary.

I'm pretty clear on the general procedure, I think: pop blister, grind out until no more water shows, wash thoroughly several times to clean, allow to dry and then apply a coat of epoxy. Fill resulting 'hole' with epoxy paste (or similar), sand smooth, another coat of epoxy (?) and then anti-foul.

Does this sound right? Have I missed anything?

There's an interesting video on Youtube that deals with the matter - does anyone have any observations on his advice? It's at this address: http://www.youtube.com/watch?v=9T_eFVF6vOw

This will be my first go at blister repairs, so I want to get it right! Advice from those who've successfully treated blisters would be really appreciated, along with suggestions for tools and materials.

Cheers.

 

[macd]

I'm pretty clear on the general procedure, I think: pop blister, grind out until no more water shows, wash thoroughly several times to clean, allow to dry and then apply a coat of epoxy. Fill resulting 'hole' with epoxy paste (or similar), sand smooth, another coat of epoxy (?) and then anti-foul.

Does this sound right? Have I missed anything?

Can't see much wrong with that. The "wash thoroughly" is important: no poncing about with a damp sponge. Belt it with a jet wash, repeatedly, over a period of time. Be patient. Incidentally, I can't see much point in the 'final coat of epoxy' over the epoxy filler.

In case it's occurred to you to barrier-coat the rest of the hull, I'd suggest not (unless, perhaps, the blisters are in a confined area). If you've got random blisters, similar chemistry is almost certainly going on elsewhere. The last thing you want to do is seal it in. But then if you've just bought her, you've probably got moisture readings for the whole of the hull.

 

[Graham_Wright]

grind out until no more water shows,
Cheers.

Wet and dry vac over the hole.

 

[knuterikt]

Have a look at chapter 4 ins this document http://209.20.76.247/ss/assets/howto-pub2/Gelcoat Blisters Diagnosis Repair and Prevention.pdf

I have successfully used plastic packing tape to shape and keep the epoxy in place while curing (less sanding)

 

[PCUK]

Grind out all the blisters and pressure wash and then leave the boat to dry for about six months in the open before treating. Have the moisture content checked at regular intervals as well to show how well she is drying.

 

[Steve_N]

Advice from those who've successfully treated blisters would be really appreciated, along with suggestions for tools and materials.

Cheers.

I did mine 9 years ago and so far haven't had any treated blisters re-occur or any new blistering.

By treating individual blisters, as opposed to removing the entire gelcoat, you are taking the decision to live with a 'wet' hull as no amount of washing and drying will get the moisture levels down or the reacted fluid out with the gelcoat in place. I would therefore ignore the recommendations to pressure wash, leave for 6 months etc. as it will just be time wasted.

You can grind-out blisters over the entire hull or do as I did and treat (grind-out, wash, repair, fill, fair) an area at a time. I chose the latter, working in circa 1 meter square patches, as it was less sole-destroying and you could see the results of your extensive labours as you went along.

A light sanding will reveal many small pimples in addition to the obvious bulging blisters and you will need to decide where to draw the line in terms of treatment. My method was to push hard on each pimple with the corner of a screwdriver and if I couldn't make an impression or hear the slightest sound then all well and good - leave them for future years.. Even the slightest 'crack' sound indicated a dry void, plus of course the obvious squirters where fluid came out!

Tools-wise a blunted countersink bit in a drill is good for the small ones and a Garryson flap disc in an angle grinder for the larger ones - be careful as they eat GRP for breakfast.

I wasn't content to just use filler on the larger craters (a 15mm blister could be 30mm x 10mm deep by the time all the affected material is gone) so spent many happy hours cutting small circles of fine GRP cloth to use to build them up to gelcoat level.

For fillers, epoxy + colloidal silica as the base then a skim of epoxy + West Lightweight (the brown stuff) on top as this is easily sanded to a fair finish.

Fun at times, sole-destroying at others, but very satisfying once done. Enjoy!

 

[PCUK]

Have to disagree with you there Steve,
I treated a Seamaster 30 with a similar problem and the moisture level in the hull dropped dramatically after a winter in the open. It's only when you seal the moisture by premature epoxy coating that the trouble starts.

 

[Steve_N]

Have to disagree with you there Steve,
I treated a Seamaster 30 with a similar problem and the moisture level in the hull dropped dramatically after a winter in the open. It's only when you seal the moisture by premature epoxy coating that the trouble starts.

Fair enough but I guess that what I meant was that I think there is little point in drying the hull if doing blister repairs.
The levels may drop, and I guess mine did over the time ashore, but if you are only doing localised blister repairs and not doing a full peel and barrier coat then what's the point of waiting for the hull to dry?
It doesn't need to be dry overall for blister repairs (use a hairdryer or heat gun locally if need be), plus as soon as you re-launch it will just soak up the water again anyway because you are treating the effect rather than the cause.

 

[PCUK]

Ah, right, When I did mine I applied an overall epoxy coating so as you say, a different matter.

 

[Witchwood]

Thanks for the responses, which are most helpful. It's clearly going to be a long haul over winter but obviously not beyond the wit of a keen amateur to deal with. If the hull remains 'wet' then I suppose it's going to be something I'll have to revisit in the future.

 

[oldsaltoz]

Can't see much wrong with that. The "wash thoroughly" is important: no poncing about with a damp sponge. Belt it with a jet wash, repeatedly, over a period of time. Be patient. Incidentally, I can't see much point in the 'final coat of epoxy' over the epoxy filler.

.

If the treated area has been faired you risk exposed fibres that water will certainly track.

Even sanding off a fairing compounds like Micro-balloons or Micro-sphere's it's important to seal the the surface. I apply 4 coats of epoxy resin over any finished area.

Good luck and fair winds.

 

[Elessar]

I did mine 9 years ago and so far haven't had any treated blisters re-occur or any new blistering.

By treating individual blisters, as opposed to removing the entire gelcoat, you are taking the decision to live with a 'wet' hull as no amount of washing and drying will get the moisture levels down or the reacted fluid out with the gelcoat in place. I would therefore ignore the recommendations to pressure wash, leave for 6 months etc. as it will just be time wasted.

You can grind-out blisters over the entire hull or do as I did and treat (grind-out, wash, repair, fill, fair) an area at a time. I chose the latter, working in circa 1 meter square patches, as it was less sole-destroying and you could see the results of your extensive labours as you went along.

A light sanding will reveal many small pimples in addition to the obvious bulging blisters and you will need to decide where to draw the line in terms of treatment. My method was to push hard on each pimple with the corner of a screwdriver and if I couldn't make an impression or hear the slightest sound then all well and good - leave them for future years.. Even the slightest 'crack' sound indicated a dry void, plus of course the obvious squirters where fluid came out!

Tools-wise a blunted countersink bit in a drill is good for the small ones and a Garryson flap disc in an angle grinder for the larger ones - be careful as they eat GRP for breakfast.

I wasn't content to just use filler on the larger craters (a 15mm blister could be 30mm x 10mm deep by the time all the affected material is gone) so spent many happy hours cutting small circles of fine GRP cloth to use to build them up to gelcoat level.

For fillers, epoxy + colloidal silica as the base then a skim of epoxy + West Lightweight (the brown stuff) on top as this is easily sanded to a fair finish.

Fun at times, sole-destroying at others, but very satisfying once done. Enjoy!

very well informed and accurate. I disagree with the people who disagree with you.

 

[Elessar]

Have to disagree with you there Steve,
I treated a Seamaster 30 with a similar problem and the moisture level in the hull dropped dramatically after a winter in the open. It's only when you seal the moisture by premature epoxy coating that the trouble starts.

the only way GRP will dry just by leaving it out is if the osmosis is so advanced that the gelcoat has split at the bubbles and at ex bubbles.

The general rule is that GRP does NOT dry if left out with the gelcoat left in place. The water that has passed through the gelcoat has taken 10s of years to get in there. It has become denser when in there by dissolving parts of the laminate. Just as water becomes denser by dissolving salt. So if it took 10s of years to get it it will take 20s of years to get out.

Quite simply, it is not coming back out unless the hull is already hopelessly damaged. Or the boat is made out of wood.

Steve though has described his experience of affecting localised repairs and accepting that elevated moisture levels will remain. It worked for him so well done for sharing.

 

[keensailor]

Fair enough but I guess that what I meant was that I think there is little point in drying the hull if doing blister repairs.
The levels may drop, and I guess mine did over the time ashore, but if you are only doing localised blister repairs and not doing a full peel and barrier coat then what's the point of waiting for the hull to dry?
It doesn't need to be dry overall for blister repairs (use a hairdryer or heat gun locally if need be), plus as soon as you re-launch it will just soak up the water again anyway because you are treating the effect rather than the cause.

In about 4 months we need to repair about 8 blisters ranging in size between 10mm and 35mm. After the repair I was planning to coat with epoxy (International Interprotect) but after reading this post I'm not sure it is a good idea, I might be "trapping" the moisture in and end with with blisters everywhere? Another guy in our club had this happen to him.
My boat is 29 years old and has never had blisters repaired. Am I right in thinking if I get these 8 done it may take another 29 years for a few others to develop (without an epoxy barrier coat). If so I am better off not using epoxy.

 

[maby]

Thanks for the responses, which are most helpful. It's clearly going to be a long haul over winter but obviously not beyond the wit of a keen amateur to deal with. If the hull remains 'wet' then I suppose it's going to be something I'll have to revisit in the future.

A wet hull is not going to sink, so the approach to take really comes down to your ultimate objectives for the boat.

You've bought it quite recently and presumably want to be sailing it, not watching it sit in the boat park for many months. If it's a potentially valuable boat that you want to restore to its original glory, then keep for many years, then it's worth doing the best possible job which will include leaving it to stand for an extended period to dry out. If it's an elderly, cheap boat that is going to be a stepping stone in your progression through boat ownership, then do a quicker job and actually enjoy sailing it, not sanding it.