Dry sand or wet sand?

[Donna]

Recently had the gunnel, boot strip and bottom painted. After being forced into a piling and turned sideways at a very sub par marina, I need to sand a couple of small patches on my gunnel and repaint. Long story. My question is, should I dry sand these areas or wet sand them? I will be doing it by hand as it does not require a power sander.

 

[alormaria]

I usually dry sand wood and wet sand metal.

Fiberglass I try not to sand - but when I do it's dry sand for the course grits and wet for the finer grits. Fiberglass I always seem to start out with a grinder so the decision is made for me. Wear rubber gloves and breathing protection. When you get fiberglass splinters use duct tape to pull them out. Don't take a hot shower until you do.

 

[Donna]

Hmm, I'm not thinking the job is that big. My thought process is what would it really matter in the spot the size of my hand if I dry sanded and it happened to rain and I wanted to finish the sanding. Seems it has been raining everyday here and if I want to get some paint on it this would have to be done around 6am. That would give the paint several hours to dry and weather won't bother it at that point.

 

[psneeld]

Dry sand before fiberglass repair, wet sand after.

 

[jleonard]

I would dry sand and then give it a quick acetone wipe just before painting.

 

[Nomad Willy]

Sorry to say it Jeff but acetone may destroy many finishes.

I'd use a much much less strong degresaer. I have found a solvent-like wash called the ultimate prep or something like that. It's specifically for cleaning while prepping for paint. I had (for decades) a very weak solvent called Doosol (Sp?) (Dupont). Got it to clean my varnished wood propeller on my UL aircraft. Mostly to wipe the bugs off but I didn't want to damage the varnish on the prop and I cleaned it frequently. Used it extensively as I found many uses for it .. including washing my hands.

What you use shouldn't flash off like acetone but should dry faster than automitive solvent. When I get home I'll send you the name of the perp wash.

 

[bayview]

Indeed many solvents will lift the remaining paint.

 

[Seevee]

Hmm, I'm not thinking the job is that big. My thought process is what would it really matter in the spot the size of my hand if I dry sanded and it happened to rain and I wanted to finish the sanding. Seems it has been raining everyday here and if I want to get some paint on it this would have to be done around 6am. That would give the paint several hours to dry and weather won't bother it at that point.

Al has it right, dry sand the courser stuff and wet sand the finer stuff, perhaps down to 500 grit.

Get yourself a supply of good sandpaper, from 80 or even courser) to 1200 for this stuff.

Assume you do not have gel coat? That's a bit different, but not hard.

And what kind of paint are you using? If you could get a sample color to your paint supplier and have them mix an exact color, you'd have it for all your touch up stuff.

And you can spray or brush most paints... even poly urethan paints, but requires a really high end brush and very light coats. Works well when spray painting is a PITA for small areas.

 

[alormaria]

When it rains, just hold your hand over it until it stops.

 

[diver dave]

Hard to talk with complete precision here, since we haven't seen the depth of the injury. But some general guidlines are useful.

Since this is a painted surface, avoid strong solvents. Very few paints will withstand direct acetone without softening. Denatured alcohol, mineral spirits, or the custom (wiping prep) work fine.

I'll assume you are dealing with cosmetic scratches. And, these will need filling. First off, get a 3M hand sanding pad; Bend and tear along a straitedge full sheets of sandpaper to fit. Without seeing the damage, I'd guess start with 120-150 grit dry paper to feather the damage into the good areas by an inch or two. So, at this point you are mostly removing good paint, and roughing the area of direct damage. Clean out the residue with alcohol. After a complete dry, you now need to fill the area with something sandable. That would NOT be marine tex or any other structural epoxy. For a fairly large job, I use West epoxy hand filled with sandable additives. For a tiny job, maybe like this, I would consider a surfacer putty or 3m Glazing putty. Interlux makes this, and I wouldn't even have a reservation using an AUTO putty. They dry fast and feather sand out nicely. Body shops use a ton of this stuff. Its fine above the waterline.
Once dry, attack with 180/220 grit until it won't catch a fingernail, then move to 440. It should be ready for touch up paint after that. Wet sanding is fine when using 220 or finer on metal or plastic. Its wood that will have grain rising issues when exposed to water.
Beyond all, remember the RULE: paints only change the color, it doesn't fill, smooth or make the surface non-wavy! It exploits all existing defects and makes its own! pinholes turn into volcanoes...

 

[Donna]

I was hoping for not a huge job. It is a few spots on the gunnell. I would like to feather it in. I have the exact paint already. It is Pettit E-Z Poxy self leveling. I have worked with this paint before. The point of even painting the gunnell was to change it from hidious blue to grand banks beige. I don't know why it was painted in the first place. I would have left it. Originally it was just white gelcoat. I just couldn't stand that blue.

So, I'm going to feather sand it, wipe it down with alcohol, roll a coat of paint on it and call it, "good enough" for now.

 

[psneeld]

Did you think the white to begin with was OK? Sanding down and re gel coating isnt much harder and better lasting than layers of paint.

But for touch up, a sime dry sanding is all thats needed. The fairing and final sanding to finish quality takes oatience and knowing whether you are leveling or polishing to a high gloss. The polishing is usually done wet.

 

[Donna]

Did you think the white to begin with was OK? Sanding down and re gel coating isnt much harder and better lasting than layers of paint.

But for touch up, a sime dry sanding is all thats needed. The fairing and final sanding to finish quality takes oatience and knowing whether you are leveling or polishing to a high gloss. The polishing is usually done wet.

The previous owner SPRAY painted the gunnell blue and didn't do a good job. There was over spray everywhere. Working on it today. It is the first thing I see when I walk up to the boat.

 

[FF]

"Seems it has been raining everyday here"

In November it will dry out,

 

[RT Firefly]

Greetings,
Ms. D. If you're trying to remove overspray, try some rubbing compound.