is barrier coat required for raising the bottom job 7 inches

[morays]

Hi,

My 40 Caliber has the top edge of the bottom paint just at the water line. If the water tanks and holding tank are near full then the edge of the bottom job is under water. For this reason there is a constant build up of algae below the boot stripe. I will be getting the bottom paint redone in the near future and will have the edge of the bottom paint moved up 8 inches to prevent this happening again. I wondered if the area that was just gel coat that will need to be painted will require a barrier coat first? The boat stays in the water all year round. Would I need to let the hull dry out at the water line before a barrier coat was applied?
If this is the case would I be better off getting the bottom soda blasted and having the whole bottom barrier coated first? I liveaboard the boat and money is definitely an object so I am looking for the best I can do without breaking the bank.
Anyone have good advise on a good shop to have this done in and around Kemah, TX?
Any help/advice would be appreciated.

Moray
s/v Sol Purpose

[neilpride]

Many factors involved, in my case i make a complete barrier coat job in my boat time ago and i barrier coat to above the waterline , some boats get pox or tiny blisters around the waterline most cases by poor resin gelcoat job in the mold, if you are going to do the whole bottom , barrier coat the waterline to, if not, depend , if the laminate is dry and we know that the bottom sunk just with the tanks full , antifouling over the sanded gelcoat, for peace of mind 3 or 4 coats of interprotect 2000 e and done..

[roverhi]

The gel coat should be all the barrier coat that you need. There are a ton of 40 year old or older boats out there that are doing just fine without a barrier coat of any kind. Sand the gel coat to give the paint some 'tooth' and slap the bottom paint on.

[Sailor Doug]

Gel coat is pours on a micro level. Blisters are cased by water penetration and a chemical reaction with resin. This reaction depends on what chemistry was used on your boat. Some old boats have done well. Chemistry was changed for environmental and financial reasons. Boats built in the eighties is when became a very big problem. Epoxy coatings have really helped to mitigate this problem.
If it is a visual concern just move the boot top up. Old race boats had boot top on design waterline so you could determine trim and displacement. I have preferred to have a extra 6" of bottom paint.
Sanding a barrier coat only helps to scratch through a very thin layer. I never use anything more aggressive than a Scotch pad.

[colemj]

If that area hasn't blistered yet, why do you think it would just because you put paint on it.

8" more? Holy crap that is a lot! We raised our bottom paint 2" a couple years ago for the same reason as you, and that has been fine for keeping the algae off.

You could paint up to the portholes - that would do it!

Mark

[Sailor Doug]

My whole long reply was to say DONOT SAND. If you do you may want some epoxy after, sooner or later.

[SkiprJohn]

In a word, "No."

[CharlieCobra]

Why have you got an extra 6,300 pounds of weight on your boat would be my question? The typical boat settles an inch for every 900 Lbs..

[robert sailor]

Give the area at and above the water line a light but thorough sanding and apply your antifouling. Add an extra coat for the first foot or so at the waterline as it gets scrubbed more. If your doing a fair amount of sailing add an additional coat in the forward bow section and the front of the keel and rudder.

[Garrettw]

Originally Posted by morays

Anyone have good advise on a good shop to have this done in and around Kemah, TX?
Any help/advice would be appreciated.

Moray
s/v Sol Purpose

South Texas Yacht Services in the Watergate Marina did my haul out and antifoul. Did a fine job as far as I could tell.

[morays]

Thanks for all the replies
@sailor Doug - your thoughts on the porosity of gel coat match up with what I have been reading. This is why I am concerned that I may have to let it dry out before carrying out any painting to get the moisture out. Would not want to trap it behind a barrier coat. If I have read your replies correctly you would not advise sanding the existing bottom boat, just Scotch pad. How about the gel coat area that will be covered in barrier coat? Light sand there first as NeilPride and Roverhi suggested for something to bond to?
@CharlieCobra - we are already below the waterline with full tanks and we dock in brackish water. We also liveaboard but have not yet loaded the boat as it will realistically be once we set off on our voyage soon (spares, sewing machine, fully provisioned for ocean crossing etc).
Worst case scenario of full boat in fresh water and a few inches of safety margin and I came up with 8 inches. I have also looked at several other Calibers in our marina and all of them have the bottom job coming up at least 4 inches higher and no one lives on them so they are not likely to be heavily loadedbe carrying much

[morays]

Originally Posted by Garrettw

South Texas Yacht Services in the Watergate Marina did my haul out and antifoul. Did a fine job as far as I could tell.

Thanks for the advise Garretw

[zeehag]

blisters are more prevalent in boats that have had gelcoat SPRAYED into molds.
the older method of pàinting it into the mold, 1/8 inch thick produced no blistering.
i learned this while watching a hull i ordered be laid up in wilmington kali in 1990-1991.
very interesting how boats are built. funny how epa changed the formula and methods of layup at same time. then is when boats began to blister from osmosis.

[Sailor Doug]

Originally Posted by morays

Thanks for all the replies
@sailor Doug - your thoughts on the porosity of gel coat match up with what I have been reading. This is why I am concerned that I may have to let it dry out before carrying out any painting to get the moisture out. Would not want to trap it behind a barrier coat. If I have read your replies correctly you would not advise sanding the existing bottom boat, just Scotch pad. How about the gel coat area that will be covered in barrier coat? Light sand there first as NeilPride and Roverhi suggested for something to bond to?
@CharlieCobra - we are already below the waterline with full tanks and we dock in brackish water. We also liveaboard but have not yet loaded the boat as it will realistically be once we set off on our voyage soon (spares, sewing machine, fully provisioned for ocean crossing etc).
Worst case scenario of full boat in fresh water and a few inches of safety margin and I came up with 8 inches. I have also looked at several other Calibers in our marina and all of them have the bottom job coming up at least 4 inches higher and no one lives on them so they are not likely to be heavily loadedbe carrying much

Epoxy takes five coats to be effective. If you do not have a problem you may try a good scrub tapping off new area and a acetate wipe down before putting on bottom paint.
I paint area between actual waterline and boot top every other year and scrub alternate years. I use ablative paint. Fortunately durning summer I am in fresh water but am looking at 5 months on the hard.

[sabray]

So many good answers but still many that are wrong. If you have a barrier you do not want water wicking below the barrier. Blister is symptom like poising ivy. Some folks never get a bad reaction and some blister. It does not mean that you are not exposed.
Means you are not showing symptoms. Glass polyester resin can dissolve without any blister.
I would raise the barrier overlap below the waterline.
V. CThink about toilet paper it absorbs water and allows that to absorbent feature to run against gravity. Glass Mat and roving may have the capacity to add to its saturation. It could absorb water and allow that to wick down. Mixing with the resin. Now it's pressing against the barrier coat. Where before it would just leach through the porous gel coat. That's why wet poor blister jobs fail.
This is pretty common knowledge but denied by many cause its inconvenient.