To rinse or not to rinse? Sanded bottom

[wwilson]

I just finished the most hated job of spring - sanding the boat's bottom as prep for a new coat of ablative paint. A new tooth filling, or even a colonoscopy couldn't be less fun!

Now - never mind my choice of tools, paint or technique - those "horses" have all been beaten to a near death experience in other threads! My question is:

What is the sense of the forum on spraying the sanded bottom after a pretty thorough sanding with an orbital vacuum sander and 80-grit paper? Some insist that a good water rinse is essential (to clear the grit) - others say don't let the bottom get wet for 2-3 days prior to applying the new paint, better yet, don't spray it at all!

I did spray it last season and this spring I noticed the last coat of paint had separated and chipped away in a few spots. Not blisters - the hull surface is fair and dry as a bone (according to a recent insurance survey). The chipping, the result of moisture from last year's paint application?

[zz4gta]

what paint is on there? Brand specific. Buy that paint thinner and wipe down the hull by hand with disposable rags. The thinner will evaporate much faster leaving a clean and dry hull to apply the new paint to. Just hosing it down isn't enough.

[deniseO30]

So... you don't want to hear the words "barrier coat" at this time? sanding = more porous then the gel-coat ever was. (just my humble opinion)

[sailingdog]

Simply spraying the hull with water to rinse it isn't going to cause problems as long as you let the water dry before applying the paint. It isn't like the bottom is all that porous or easily saturated with water.

If just the last coat of paint was chipping, it isn't because you rinsed the hull. If the paint was going to flake off because the hull was rinsed, ALL of the paint would be chipping or blistering, not just the last layer. Think about it—how can the rinsing of the hull affect the last layer but not affect the layers between the hull and the last layer... does that make any sense at all???

BTW, if you are sanding down to the gelcoat.... I'd second Denise's suggestion of barrier coating... but the hull has to be dry for you to do this.

[tager]

I would wipe it down with a clean rag and some solvent to get every last bit of goo, wax, and dust off, before putting on any coating, paint, barrier coat, or otherwise.

[wwilson]

Originally Posted by sailingdog

Simply spraying the hull with water to rinse it isn't going to cause problems as long as you let the water dry before applying the paint. It isn't like the bottom is all that porous or easily saturated with water.

If just the last coat of paint was chipping, it isn't because you rinsed the hull. If the paint was going to flake off because the hull was rinsed, ALL of the paint would be chipping or blistering, not just the last layer.

sailingdog,

I'm not as sure. The chips were over prior years paint. When the chip appears, there is still one (or more) coats of paint underneath - not bare gelcoat. Furthermore, most of the chips were near the waterline - on that portion that gets (intentionally or inadvertently) more paint than the rest of the hull. This is not to argue with your point, but better describe my own.

I suspect the rinse left moisture on the old ablative (and relatively porous) paint. When the new coat was applied it dries as expected, but did not achieve adequate adhesion to the surface, particularly in those spots with some amount of moisture still present though not visibly detectable. To your question, why one layer and not all? My explanation is that the prior year's paint achieved proper adhesion, the last layer could not because of small amounts of moisture within the base ablative paint caused by the water rinse the day prior to painting. What say you to that: reasonable possibility, or pure bunk?

[sailingdog]

Obviously you do still have to let the bottom dry after rinsing it. However, this shouldn't take too long. It sounds more like the previous paint job was done in a hurry and applied before the hull had a chance to dry off completely.

For the paint to absorb any significant amount of moisture takes prolonged exposure to water, and a quick, brief rinse to get rid of the sanding residue shouldn't allow it to absorb much at all. If you are still leery of using water, you could always wipe down the hull with something like acetone or denatured alcohol, which would allow you to remove any sanding debris, clean off any wax/grease and would help pull out any residual water in the bottom paint as well.

Originally Posted by wwilson

sailingdog,

I'm not as sure. The chips were over prior years paint. When the chip appears, there is still one (or more) coats of paint underneath - not bare gelcoat. Furthermore, most of the chips were near the waterline - on that portion that gets (intentionally or inadvertently) more paint than the rest of the hull. This is not to argue with your point, but better describe my own.

I suspect the rinse left moisture on the old ablative (and relatively porous) paint. When the new coat was applied it dries as expected, but did not achieve adequate adhesion to the surface, particularly in those spots with some amount of moisture still present though not visibly detectable. To your question, why one layer and not all? My explanation is that the prior year's paint achieved proper adhesion, the last layer could not because of small amounts of moisture within the base ablative paint caused by the water rinse the day prior to painting. What say you to that: reasonable possibility, or pure bunk?

[wwilson]

Thanks all,

I will keep the water away from the bottom this year until well after the new ablative is applied (then Bay water only).

I suspect moisture from last year's application. If it bubbles again this season my guess will have been wrong. If it doesn't bubble up again - I may not be able to conclude moisture was the culprit - but I'll be happy anyway.

[hellosailor]

ww-
Problems with paint usually come down to mixing products that are incompatible, or not following the manufacturer's instructions. When you've got an old boat and you have no idea what the layers on the bottom are...You can either strip it down to the gelcoat and start fresh (which you may have to do if it is dropping off) or put on a primer/barrier coat that is designed for the best adhesion over "mysteries" and designed to work with the new paint that you want to apply.
Whatever you plan to apply--see what the manufacturer says about primers, wet sanding, drying time, etcetera. There's usually some method in their madness and most of them will answer the phone to discuss these things. (Heck, at those prices they can afford to answer collect calls!)

If you plan to keep your boat...it might just be worth trying to get rid of as much old paint as you can, until you know "for sure" what you are putting the new paint on top of.