What method of sanding before new paint?

[mspring]

My hull is chalky and has numerous spider cracks. I want to sand down and repaint topside, bottomside, and deck. I am going to buy an electric sander.
Should I buy a orbital or palm sander? I dont think it would make a difference on the old gel coat but what about in between coats?

Also I made the mistake of filling some spider cracks and holes with automotive bondo Should I sand it out and use marine-tex?

 

[rockyrude]

Re: What method of sanding before new paint?

One thing at a time, yes get as much of the bondo out as you can, it doesn't really stand-up to the flexing and will eventually pop off. As far as sanding, you might want to look at wet sanding, there will be a lot of dust otherwise. Use a 220-320 grit to prep for paint. Depending on your top coat, you can use 320 between coats. For the decks, add the no-slip additive. If you want to go power, a palm sander won't tire the arms so quickly.

 

[mspring]

Re: What method of sanding before new paint?

Do you mean an orbital sander will tire your arms out more than a palm sander?

Also I dont know if I have seen sanding pads for wet sanding available for power sanders. Does it take special sand paper?

 

[rockyrude]

Re: What method of sanding before new paint?

You didn't say how big the boat is but the extra pound the orbital weighs will ruin your day in a hurry ( gets the job done faster ). You said you were doing the hull also, think about which sander you want to be using on your back doing the bottom. I have done wet sanding with my sanders, just used regular wet/dry sandpaper, just got to be careful with the water.

 

[Chris1956]

Re: What method of sanding before new paint?

You are going to need to wet sand the hull. I found some course sponge-backed wet paper in the 80-100grit range. I used that on my palm sander, and used a hose, GFI outlet and rubber shoes. gradually graduate ot finer paper.

When the sander starts to tingle, it is time to dry it out for a while. Air powered sanders are better for wet sanding, BTW.

 

[mspring]

Re: What method of sanding before new paint?

Why do I have to wet sand the hull?

 

[Chris1956]

Re: What method of sanding before new paint?

On my hull the paint would melt and clog the paper, when dry sanding. Wet sanding kept the paint cool and allowed me to remove it. Wet sanding is also the only way to get the fine finish that 400 grit paper provides.

 

[ianbze31]

Re: What method of sanding before new paint?

A couple of quick clarifications. Is you hull wood, metal (aluminum) or fiberglas? To what level of restoration do you intend to take this project?

If you simply want to "recolor" the hull, you don't need to take off all the undercoat - just enough so that your color coat and clear coat will adhere well.

Also, how big is your boat and will it be runing in salt or fresh water?

The more info you can provide, the more appropriate the advice.

Cheers.

 

[rockyrude]

Re: What method of sanding before new paint?

I'm with ianbze31, we really need more info, I presumed that since you said you had spider cracks you had a fiberglass boat.

 

[mspring]

Re: What method of sanding before new paint?

Thanks for all the replies. It is a fiberglass 20 footer. Topside really chalky and bottom well on its way with a lot of deep scratches near keel. Used in salt water in SW Florida. I just want to repaint it white and get a shinny hard finish.

Should I just sand out the spider cracks and fill with marine tex?

Also would someone please explain the method of wet sanding? Do you actually spray the surface with the water hose then sand?

 

[Chris1956]

Re: What method of sanding before new paint?

Normally to wet sand, you dip the paper into the water, sand a while and then redip the paper. Obviously if the paper is on a machine you use a hose to give it some fresh water periodically.

 

[mspring]

Re: What method of sanding before new paint?

Thanks Chris. Looks like I may have to sand by hand, not looking forward to that. What would you do about the spider cracks?

 

[Sorrento 25]

Re: What method of sanding before new paint?

Marine-Tex would be best, but it is expensive and difficult to work with. Since you are recoating the repairs anyway I would recommend a chopped fiberglass or aluminum reinforced filler. Prep the hull for primer dry and with your powered sander but after primer application I would work by hand wet. It is way too easy to break through and have to recoat areas when sanding a relatively thin application of primer with a power tool on all but the flattest of surfaces. A guide coat will help quite a bit too.

 

[mspring]

Re: What method of sanding before new paint?

Thanks Sorento. After I thought about it the paint is so chalky, especially topside, power sanding will probably not be a problem. The spider cracks are really hairline cracks. Would I still use a filler with hair fiber?

You said a guide coat would help.What do you mean by a guide coat?

 

[Tail_Gunner]

Re: What method of sanding before new paint?

Dry sanding=dust
Wet sanding no dust

If your gel has fine crack's you need to sand them out, they will continue cracking just like a glass winsheild in a car. I would use no more than 180 grit wet or dry, if done properly that grit can cut almost anything. Wet sanding will allow you to rinse the paper free of any build up which will happen quite a bit on old chalky gel.

When your done if you apply Interlux prime-kote as a primer it will help stablilize your old gel and help prevent any more crack's.........it's a extremely hard primer.

 

[mspring]

Re: What method of sanding before new paint?

Thanks TailGunner very good info. What would you recommend for filler after I sand the cracks out?

 

[Sorrento 25]

Re: What method of sanding before new paint?

Generally I reserve wet sanding for finer grits and post-primer application prepwork. You certainly could sand wet all the way through the project but if you are going to be opening up gelcoat cracks to raw fiberglass you are better off working dry until you have everything sealed up with primer IMO. A fan and a hand broom will help keep the work area and paper clean. Wear a mask of course.

Guide coat is just a light mist of a contrasting color applied over the last coat of primer. When sanding wet it is difficult to see your progress, and this guide coat allows you to sand just enough and not too much.

 

[mspring]

Re: What method of sanding before new paint?

Thanks again tailgunner. I think I am almost ready to begin.

 

[mspring]

Re: What method of sanding before new paint?

Thanks also Sorento good info.

 

[Tail_Gunner]

Re: What method of sanding before new paint?

Thanks TailGunner very good info. What would you recommend for filler after I sand the cracks out?

Interlux make's a fairing putty, good stuff.........take a magic marker circle all the area's you plan to fair..........make a plan..........mix no more putty than 5 min's worth it dries very fast...........do the job ............and have a beer..............