Blisters - How deep is enough?

[Sailshabby]

I’ve found some blisters on my hull and am in the process of remedying the situation with some good old fashioned 50 grit and fentanyl to get through it. (Just kidding about the fentanyl… at this point!)

PO had the work done once but bless his heart, it appears he didn’t pay too much attention to the job and they’re back. Most are pimple sized but the moisture meter doesn’t lie, so I’m taking it all down again. I see a tiny bit of weeping from the small ones (see photo) as I sand down but I’m not sure how much to dig once they’re exposed. Boat will be out of the water for the foreseeable future so there’s plenty of time to dry out. I plan on pressure washing several times throughout the process.

So… anyone have any (successful) experience here? Cut down until everything sands dry? Some tiny weeping is okay because it’ll was out and dry before the barrier coating begins?

Thanks, folks.

[Jeff on Aurora]

Good morning!

I did an extensive blister job 30 months ago in Key West (high humidity environment; no real chance for the hull to "dry".) I just pulled out my boat at Bock Marine yesterday for bottom paint and thankfully found no new blisters. (Huzzah!!!)

Here's what worked for me:

1) Wet-blasted (sand) the hull down to the original gelcoat. Note: prior owner had not barrier coated the boat, hence the blister problem.)

2) During early morning/late afternoon when sunlight was raking the hull at a low angle, I looked for bumps that indicated blisters and circled them with a Sharpie. I had thousands - ugh!

3) Tried several grinding methods:
a. Dremel (too tiny, slow, not robust enough for the job, no depth control other than careful handling)
b. Angle grinder (too aggressive! not precise enough)
c. Various sanders / various grits 40/60/80 (not precise enough, sandpaper very expensive for this big job)

d. ...and the winner... Small Makita laminate trimmer/router with a 1/2" round bit. Precise, easy depth control, lightweight - I spend 9 full days grinding)

I set the depth to about 1/8" in order to dig down to good dry glass. Of course every boat and damage situation is going to be different. I set the depth based on advice from a 30-year experienced fiberglass and paint professional. He told me to "keep digging" until I didn't see any more weeping or color change.

4) After grinding, I used a pressure washer to rinse off grinding dust and the acetic acid pus that had built up in the blisters. After "drying" for a day (as much as possible in 80% humidity) I power washed a second time.

5) All cratered out blisters were wiped down with the "two rag" method and acetone.

6) Blisters were filled and faired with AwlFair.

7) Six coats of barrier coat were applied, then bottom paint.


Whew! This was a six week project on an Island Packet 38 - lots of wetted surface on this chubby girl!



So, there's a long answer to a short question, lol. I guess the short answer is the one that was given to me by the professional:

"Keep digging"





Best regards,
Jeff

[Gary Mc]

It depends on your hull, if it is one inch or so of solid glass then you can afford a to take a little of the laminate off. If it is cored your outer skin may not be very thick.
What I did was using a random orbital took almost all of the gel coat off but left traces of it to ensure I did not go too deep and lose some strength.



Have fun!

[Sailshabby]

Thanks, Jeff. Your path is exactly mine. I initially went with the dremel and their cutting wheel, then progressively coarser orbital sanding. I read somewhere about someone using your router trick and that might be the solution. Since you’re at Bock, which is right down the road from me, how about stopping in and showing me? Gotta warn you I’m a slow learner and it could take weeks of watching you before I catch on…

[Sailshabby]

Sorry - 1/2” round nose router bit?

[Iron E]

Originally Posted by Jeff on Aurora

Good morning!

I did an extensive blister job 30 months ago in Key West (high humidity environment; no real chance for the hull to "dry".) I just pulled out my boat at Bock Marine yesterday for bottom paint and thankfully found no new blisters. (Huzzah!!!)

Here's what worked for me:

1) Wet-blasted (sand) the hull down to the original gelcoat. Note: prior owner had not barrier coated the boat, hence the blister problem.)

2) During early morning/late afternoon when sunlight was raking the hull at a low angle, I looked for bumps that indicated blisters and circled them with a Sharpie. I had thousands - ugh!

3) Tried several grinding methods:
a. Dremel (too tiny, slow, not robust enough for the job, no depth control other than careful handling)
b. Angle grinder (too aggressive! not precise enough)
c. Various sanders / various grits 40/60/80 (not precise enough, sandpaper very expensive for this big job)

d. ...and the winner... Small Makita laminate trimmer/router with a 1/2" round bit. Precise, easy depth control, lightweight - I spend 9 full days grinding)

I set the depth to about 1/8" in order to dig down to good dry glass. Of course every boat and damage situation is going to be different. I set the depth based on advice from a 30-year experienced fiberglass and paint professional. He told me to "keep digging" until I didn't see any more weeping or color change.

4) After grinding, I used a pressure washer to rinse off grinding dust and the acetic acid pus that had built up in the blisters. After "drying" for a day (as much as possible in 80% humidity) I power washed a second time.

5) All cratered out blisters were wiped down with the "two rag" method and acetone.

6) Blisters were filled and faired with AwlFair.

7) Six coats of barrier coat were applied, then bottom paint.


Whew! This was a six week project on an Island Packet 38 - lots of wetted surface on this chubby girl!



So, there's a long answer to a short question, lol. I guess the short answer is the one that was given to me by the professional:

"Keep digging"





Best regards,
Jeff

Thanks for putting that post together. A lot of good information