Boat Bottom Blister Repairs By Capt. Douglas Malat

Boat bottom blisters look like just that, a blister. Multiple blisters look like a bunch of pimples and they might be in various sizes. Some of these boat blisters can get the width of a softball and stick out from your boats bottom quite a ways. Blisters don't care if you have aPowerboat or Sailboat, and it can get pretty ugly if not taken care of at early stages, but if you do catch it early, (which you usually can) there’s no big deal about it. I know it might seem like a big deal, when you see all those boat blisters/pimples when you haul out, but it really isn’t. You can really handle this, just take it one step at a time…no problem.

Boat Bottom Blisters form basically from water/moisture being absorbed under the gelcoat where an air pocket/space might have been. If there is raw fiberglass fibers, it will also move into that…. There are many reasons. As more water in an area gets absorbed, its pressure gets greater than the surrounding water and bulges out, thus a boat blister. Many boats and yachts in the northern waters don’t have this problem as much because of the temperature of the water and one other factor. They haul-out for the winter months, giving the bottom time to dry out. They get hauled Oct-Nov and don’t go back in till April-May. This gives the hull bottom 4 to 5 months to dry out, so the water absorption stays in check. Another way a boat gets all those small blisters and pimples all of a sudden is when they haul theirboat oryacht out on a hot sunny day. The water that has been absorbed under the gelcoat expands quickly from the heat, and out they come. I’ve seen this myself in Ft. Lauderdale, Florida when a boat was being hauled out. The boat had black bottom paint and a perfectly smooth bottom. As it was still in the slings, the pimples/blisters like chicken pox started appearing! Some small, some bigger, but a whole bunch. Funny, it was just in the area where the bright Florida sunlight was hitting the boats bottom. It was the sun heating up the water beneath the gelcoat. Now let’s get to repairing these…

Basically if you just have a small area of them, or a few here and there, this is one way to repair them. For more extensive areas or real large blisters that ooze with gooey liquid, I’ll have to do another blog if visitors are interested on the repair….Email meat info@yachtauthority.com or leave it in the “Comment” area below.

First I would take chalk or a crayon and circle the boat blisters. Sometimes a few will disappear which you would think would be good, but in the long run, why let it fester? You see it, know where it is, and you’re doing the others, so do the ones that disappear. Pop the blisters/pimples by using a pick, punch, flathead screwdriver or a small narrow chisel to scrape the gelcoat off and expose the fiberglass bottom under the gelcoat. This lets any water out.

Next wipe away any water/liquid, and using a grinder or good orbital sander with 60-80 grit sandpaper grind/sand away any loose material on, in, around the boat blister/pimple (remember to use a mask and read all instructions and cautions on anything you use). You will know when you got it all cleaned up well with the grinder/sander…don’t go wild, these small ones (up to about ½ inche) don’t need that much grinding and sanding. Once the area looks uniform with no loose material or discoloration, that should be fine. Don’t dig in too much, you don’t need to. Take a screwdriver and pick around where the blisters/pimples were to see if there are any hollow spots.

Now the hard part…Drying time! This would be great if you can leave it for a few months out of the water like up north. If you don’t have the time (month or more) to leave it to dry, you can accelerate the process by adding heat, like heat lamps or flood lights that give off a uniform warming heat…nothing that becomes too hot to touch. This will get the moisture out, and a moisture meter will give you the information you want…one of those small great gadgets! You and I know you’ll only give it so much time to dry, but the longer the better…for the whole hull a matter of fact!

Wipe the areas with a moistened rag with acetone, clean the areas well. To fill in any irregularities, make-up a filler using resin and those micro fillers from anyMarine Supply or on Yacht Authority.com (Leftside Menu Bar). Thicken your resin to almost a paste, remember to use the hardener and apply with a putty knife. On bigger depressions, use a little bit of filler at first to get all the way in, leaving no air spaces. Then add more to level it out to the original surrounding surface. Again you can tape the areas off (readBoat Fiberglass Tabbing), so to keep your working area tight which will mean less to sand. Let your filler dry, sand with 180 or 220 grit sandpaper, using an orbital sander or hand block sander depending on the area to be done. If you need it smoother, finish with 320.

It is amazing, but you’re done…easy and you just added more value to your Boat!…and if you have larger areas, you can do it in sections so you don’t get overwhelmed… It would be good to use a barrier coat product before putting on your bottom paint…it makes it harder for the water/moisture to get back in, especially if your boat or yacht stays in the water year round…..plus since you did such a nice job… Why Knot?