is barrier coat required for raising the bottom job 7 inches

[minaret]

Originally Posted by 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.

This is so not true. Please explain to us the mechanics of why brushing gel in the mold vs spraying has anything to do with blistering (hint:it doesn't). In fact brushing introduces a whole new set of issues which are far more likely to cause trouble than spraying.

[colemj]

Originally Posted by 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.

But I think the point is that the OP simply wants to paint higher up the waterline to prevent scum, not because his current waterline is below water.

If he has not experienced any blisters after so many years of just gelcoat, I don't think painting over the gelcoat is going to make anything worse.

We have done that for the same reason, without any blistering problems.

I think barrier coating is unnecessary here, but on the other hand, it would probably only require a $70 gallon of epoxy and a bit of time.

It is also unclear whether the OP's boat has any barrier coat at all on the bottom. If not, then barrier coating the waterline addition is definitely a waste of time.

Mark