Prepping new fiberglass

See our FAQ and tutorials before posting.
Rranger
New Poster
New Poster
Posts: 6
Joined: Fri Nov 27, 2020 7:03 pm
Location: Grand Forks BC

Prepping new fiberglass

Post by Rranger »

Hello. What grit progression of 5' orbital air sander discs would I use on new fiberglass to ready it for gelcoating. In other words start with what grit and end with what grit? I need to order discs and don't want ones I don't need.


00.jpg
thumbnail (3).jpg
thumbnail (4).jpg



fallguy1000
Very Active Poster
Very Active Poster
Posts: 7123
Joined: Tue Jan 12, 2010 12:25 am

Re: Prepping new fiberglass

Post by fallguy1000 »

My boat build is here -------->

viewtopic.php?f=12&t=62495

User avatar
mattkab
Frequent Poster
Frequent Poster
Posts: 99
Joined: Thu Feb 25, 2010 8:18 pm
Location: Washington State
Contact:

Re: Prepping new fiberglass

Post by mattkab »

I start with 40 grit on the random orbital and hand sanding -- it knocks down the big lumps quickly, and I have found it to be very effective to get a first-round of fairing. I just ordered a scraper to try, based on feedback on another thread too. I use 100 grit on the belt sander because anything more aggressive can eat through to dry wood too quickly, especially on edges. I don't use a grinder personally, though I know others swear by them.

But I'm a simple hobbyist, and don't have thousands of hours of experience. And honestly, I'm not real careful with my layups.
Matt.
Seattle, WA.

GV-13 Build thread & Questions
Previously completed V10 & Launch

VT_Jeff
Very Active Poster
Very Active Poster
Posts: 908
Joined: Fri Oct 25, 2019 6:32 pm
Location: vermont

Re: Prepping new fiberglass

Post by VT_Jeff »

I'm gonna ask a dumb question: are you actually going to be sanding the glass? I normally try to avoid sanding actual glass. After glassing I fill the weave with fairing compound or neat epoxy or whatever the mood strikes, and then sand that to some finish while trying not to sand into the actual glass. I start with 60 or 80 depending on how thick/rough the fairing layer is and go up to maybe 300-400, then use a skim coat of expoxy to seal it and fill any pin holes, then sand that neat coat up to whatever level is called for by the paint. I wouldn't think you'd need to get it diamond smooth if you're going to put on gelcoat but I have not applied gelcoat so I can't say about that.

Not an expert, just sticking my nose in.
There are only two seasons in Vermont: boating season, and boat-building season.

Rranger
New Poster
New Poster
Posts: 6
Joined: Fri Nov 27, 2020 7:03 pm
Location: Grand Forks BC

Re: Prepping new fiberglass

Post by Rranger »

I am not a expert either which is why I ask questions on these boards. Its first stage basically knocking down highpoints. I just need to clean up rough spots on the back of the transom and inside and around the new motor well, strictly for acceptable looks, I'm not looking for boat show quality. I have 80 grit and that's going to have to do it. I can put two or three coats of gelcoat on the back of transom to help smooth any minor blemishes and my plan is something like tough coat on the entire floor, insides up to gunnel and the inside of transom and motorwell.

VT_Jeff
Very Active Poster
Very Active Poster
Posts: 908
Joined: Fri Oct 25, 2019 6:32 pm
Location: vermont

Re: Prepping new fiberglass

Post by VT_Jeff »

Rranger wrote: Fri Apr 02, 2021 9:13 am I am not a expert either which is why I ask questions on these boards. Its first stage basically knocking down highpoints. I just need to clean up rough spots on the back of the transom and inside and around the new motor well, strictly for acceptable looks, I'm not looking for boat show quality. I have 80 grit and that's going to have to do it. I can put two or three coats of gelcoat on the back of transom to help smooth any minor blemishes and my plan is something like tough coat on the entire floor, insides up to gunnel and the inside of transom and motorwell.
80 grit will work, I use boxes of it! If the epoxy has had time to cure to good-n-hard then it will be a breeze. If it's at all green it can be a it more of a chore. Keep that crap out of your lungs in either case, but esp if it's at all green.

I've never used gelcoat, interested to hear how it is to apply.
There are only two seasons in Vermont: boating season, and boat-building season.

OneWayTraffic
* Bateau Builder *
* Bateau Builder *
Posts: 1101
Joined: Sat Jan 10, 2015 7:13 pm
Location: New Zealand

Re: Prepping new fiberglass

Post by OneWayTraffic »

80 grit is ideal for scratching epoxy, Gougen explicitly call it out as prep for new coats of epoxy. If I have large drips, lumps and things to clear off I always start with the grinder and use a coarser grit, 26 works fast and the discs last forever. I handle the grinder very lightly as you can easily do a lot of damage fast. Then go over with a multitool and carbide rasp for corners and holes, then wire brush for weave and 80grit for surfaces. Obviously this is not for final fairing.

User avatar
Evan_Gatehouse
Very Active Poster
Very Active Poster
Posts: 3139
Joined: Tue Aug 27, 2002 1:00 am
Location: Vancouver, BC, Canada

Re: Prepping new fiberglass

Post by Evan_Gatehouse »

You did say applying gelcoat. Gelcoat does not stick to epoxy well. Well cured epoxy (a few weeks) gives better results but it's inconsistent.
designer: FB11/GV10,11,13/ HMD18/
SK17,MM21/MT24

Post Reply

Who is online

Users browsing this forum: No registered users and 2 guests