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Old 08-14-2008, 07:54 PM   #1
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Default What grit sandpaper for fiberglass hull?-prior to primer

I'm sanding down my hull and will be painting the entire thing. I was wondering what grit of sandpaper I shoud use just before priming? I used 80 grit on the transom and it just doesn't look smooth enough. So before I go screwing up the rest of the boat, what do ya'll suggest?

Thanks!:usaflag
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Old 08-14-2008, 09:17 PM   #2
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Default RE: What grit sandpaper for fiberglass hull?-prior to primer

I would wet sand it with about 800 grit,,,,
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Old 08-14-2008, 10:53 PM   #3
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Default RE: What grit sandpaper for fiberglass hull?-prior to primer

Quote:
Sequoiha (8/14/2008)I would wet sand it with about 800 grit,,,,


Ah' He's going to be priming it Kenny not polishing it.



I use 80gt on the DA and then put 4 coats of Awlgrip 545 on it. Then it get's hand blocked after a guid coat is put on the hull with 400 wet.



Examples of guide coat or "Tiger Stripes" as I call it.


















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Old 08-14-2008, 11:17 PM   #4
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Default RE: What grit sandpaper for fiberglass hull?-prior to primer


I use 80gt on the DA and then put 4 coats of Awlgrip 545 on it. Then it get's hand blocked after a guid coat is put on the hull with 400 wet.

Examples of guide coat or "Tiger Stripes" as I call it.



X-Shark,

That's a good pointer, thanks. I am painting a 1972 (or 1987 according to title)14' Starcraft fiberglass bass boat. Solid Hull and transom, just needs lots of lovin'. Heres a pic of it right after I got it (Thanks Sean). I didn't have enough money to buy a new boat so I got this fixer upper that I can slowly restore as the ends are available. It's working out great so far.

So this being my first boat painting experience, what do you suggest as far as the brand of primer and paint? I was hoping to find a primer that I can apply either by roller or aerasol, and I have access to a paint shop where my buddy (who paints cars) is going to help me with the paint gun when the time comes to paint. I'm just not sure which paint is compatible with which primer.




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Old 08-15-2008, 08:58 AM   #5
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Default RE: What grit sandpaper for fiberglass hull?-prior to primer

X-shark,

I am curious why you need that much tooth on a boat finish when 220 grit should be plenty for proper adhesion? I am not trying to challenge you in any way, rather I am trying to learn the reasoning behind using such a coarse grit followed up by so many probably heavy coats of primer and then hand blocking with the exact same grit that most bodymen would use to prep the surface for sealing/painting after a 220 rubdown. I was taught and I also taught my autobody students that the coarser grit scratches would have a greater tendency to swell and come back in the final prep to bite you in the butt with scratch marks and other problems in the final finish, particularly if you were using any 2 part epoxies or catalyzed enamels. There are some chemical actions that occur between the undercoats and the top coats of most finishes and, if they are done correctly, will ensure a quality top surface that will be pleasing to the eye and very durable to the elements. If the application steps are not adhered to, in some cases, the finish will craze, swell, sometimes crack, and you will fight sandscratch swelling until the cows come home!!

This wasn't just hearsay but was part of the required curriculums that I had to attend periodically that were put on by PPG, DuPont, Ditzler in their plants in Michigan, and a few other quality automotive finish developers located across the Midwest. I had to stay abreast of all the latest trends and products that came out of R&D in order to keep my students up on all the latest techniques that they would run into in the bodyshop when they graduated.

What I am thinking now is that, since I have been out of the trade since the early 90's, the trends have changed so much and the products have improved so radically that our "old" methods are not viable anymore. But, when I read your description about using 80 grit on a fiberglas finish, I cringed to think what some of my really picky clients would have done to me had they come into my bodyshop back in the 70's and 80's with their vintage 'Vettes andcaught me sawing away on their precious finish with 80 grit!!! :hoppingmad

I happened to be one of 4 restorers/refinishers back in the late 70's that David Burroughs of Bloomington Gold fame would recommend for restoration of Corvette bodies in the state of IL. I was South of Bloomington and had a background in working with fiberglas and special finishes and I wasn't afraid to touch one of the rare "Vettes" with a sander or grinder when needed. But the technology of finishes was progressing so rapidly that it was difficult to keep up with the trend of new techniques and the introduction of new 2 part finishes. I spent more time and money learning new techniques and products than I did sometimes using the ones that I already paid to learn!!!

So my guess now is that you are using some technique that is newer than what I was accustomed to using back "in the day". BTW, "guide coating" is one of the old tried and true methods used to instantly spot the most minor of imperfections in the final surface prior to painting. I was taught to guide coat as one of the very first steps in learning bodywork back in 1957 when I first started out in a bodyshop! I can see just from your few photos that you have more than a layman's background in refinishing as your tape and masking techniques and the guide coat application reflect the training that you obviously have in the paint field. No Walley World masking tape and newspaper on THOSE boats!!
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Old 08-15-2008, 10:44 AM   #6
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Default RE: What grit sandpaper for fiberglass hull?-prior to primer

Quote:
what do you suggest as far as the brand of primer and paint? I was hoping to find a primer that I can apply either by roller or aerasol


I already suggested the primer. Awlgrip 545. That primer is a true epoxy primer and doesn't shrink. 4 coats are sprayed [unthinned] It's plenty thin enough to go thru a 2.0 tip that I have in my primer gun and others I know in the trade do the same thing.



Quote:
I am curious why you need that much tooth on a boat finish when 220 grit should be plenty for proper adhesion?


220grt on a DA = the same as 400gt hand blocked. A DA erases it's own scratches. That is fine for large boats where your not going to block the primer.



Awlgrip recommends 80gt to 180gt for sanding the gellcoat in prep for primer. I use 4 coats of the 545, so I don't have to spend all my time repriming when blocking it out. You know most of it gets cut off and wasted when sanding, but it makes the surface straight.



Remember, Your not painting cars now. There is lot's of friction from the water and you need a good tooth.



If you prepped a boat the way you would a late model car no a days it will come off in sheets.



Regular Awlgrip is not forgiving. You can't go back and color sand it and buff it. If that is what you want to do, use the 545 primer and paint it with Imron.



Step's before sanding for primer that I do.



Wash the boat real good with dishwashing liquid. Something that cuts grease. Then was it with Wax & Grease Remover [WGR] using the 2 rag method.

Then with a pencil [NOT a Pen] go over the whole boat with a fine tooth comb and circle all the gouges and nicks.

Then you can start sanding with the DA while having a pencil in your pocket. When you come to a circled spot. Sand it and circle it again.

This is so you don't overlook the blems in the hull. It's easy to do after it's sanded if you don't mark them again.



Then the blems git filled. I use System 3's Quik Fair epoxy filler.

It will look like this when your done filling. This is the old Quick Fair. It's kind'a Purple looking. The new stuff is Tan and covers better.











For real little nicks I use a single edge razor blade as a spreader.






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Old 08-15-2008, 10:53 AM   #7
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Default RE: What grit sandpaper for fiberglass hull?-prior to primer

Here's one that was a true balancing act to paint the bottom. This belongs to my buddy Mike. It is a custom 25ftr.







You can see all the scafling that I had to climb on here.

















Flipping Party







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Old 08-17-2008, 07:10 PM   #8
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Default RE: What grit sandpaper for fiberglass hull?-prior to primer

Thanks for all the prep advice ya'll, I learned a lot from it. And X Shark, the pics were great, really opened my ideas to the process of prepping and painting.

:usaflag
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Old 08-18-2008, 03:04 AM   #9
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Default RE: What grit sandpaper for fiberglass hull?-prior to primer

Glad to help.




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Old 08-18-2008, 11:46 AM   #10
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Default RE: What grit sandpaper for fiberglass hull?-prior to primer

Yeah, thanks for clearing up those details about water drag on the hull also. I never would have considered that as a factor in prepping a boat hull but you are so right, it does become a factor even with vehicles when they are geared to run at over a hundred miles per hour also! Just the drag of the air on a surface and the impact of normal debris on that finish can peel it away if it isn't done to specifications. You wouldn't think that a June bug, for example, could pose a danger until you see what it can do when it hits a finish at 130 mph!!!

I figured that, with the advancements in refinishing products that was taking place back in the late 80's and on into the new millenium, that I was out of the loop with knowledge of what is available to the refinisher today. There was so much happening so fast back then that it was difficult to keep up with all the new products in the prep and refinishing field so I can imagine what it is like today.

Hey, it looks like you are on top of your game with the gun also. I can relate to that feeling when you feel that extra "sense" to know the difference between putting on so much as to cause runs and getting that perfect layer on that will flow out to a glass smooth finish without further work. That is a really cool feeling. I couldn't lay down a coat today to save my soul like I could when I was at the top of my spraying game!!

Nice job and a really educational reply. Thanks alot for the information.
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