What is the best method to smooth rough fiberglass

[Daleford Senior Member Find all posts View Profile Close Daleford]

I am building a "one of" product by covering a surface with 3 layers of 1 & 1/2 ounce fiberglass. I will need to smooth that surface out so that I can paint it with an automotive paint. The rough fiberglass surface has the proper shape but I need to fill it somewhat before I start sanding it down. I plan to sand with 40 grit, then 80 grit and follow with a high build primer surfacer.
I was wondering about thinning Bondo with resin? Good idea??

[powerslave Junior Member Find all posts View Profile Close powerslave]

Re: What is the best method to smooth rough fiberglass

You could try putting an extra coat of straight resin on it instead of mixing it with bondo. Depending on how thick you want it to be, you could just pour it on also. I dont know how resin would work if you mixed it with bondo, but I would experiment on spare parts before you applied it to your good part just in case it has bad results... it might not harden or it might not adhere very well and crack off in 24 hours, who knows. I'd be interested to see how it works. There's also super heavy duty thick primers that you can spray on that has a high build up amount and after a few coats, it's almost like a skim coat of bondo.

[MacGyver Senior Member Find all posts View Profile Close MacGyver]

Re: What is the best method to smooth rough fiberglass

Sand down with around 80 grit and use a Resurface Polyester primer like U-Pol Reface Polyester, Evercoat Feather fill, etc. You will also need a large tip to spray it since it is so thick. Don't think about thinning bondo. It is full of tallac and it will soak up moisture like a sponge and the paint will fall off.

[Don Senior Member Find all posts View Profile Close Don]

Re: What is the best method to smooth rough fiberglass

Dale;

I agree with you on the 40 then 80 grit sandpaper to start. Even 60 to start is quite aggressive from my experience. Go up to 120-150 or so for a final rough sand before putting your next coats on.

Once you have the basic shape in place to get rid of the last of the valleys you should put a skim coat of bondo on the full surface and sand that off with 180 or so grit paper on a long block to smooth out the surface. From there, put the high build primer on and do your final long blocking with successive high build primer coats to get the final surface you are looking for. I am doing this with my current 308 front hood and panels they are coming out very nicely with very flat surfaces.

I have put resin with bondo before to get a better flowing material and it never seems to work out very well. I have a new bondo product that was recommended to me by a long time body guy and went to my fiberglass store to get it. It comes large diameter squeezable tube rather than a bucket and applies extremely well and when sanding off, sands off like talc. Really nice stuff to use. It comes out as a really light green and is a bit thicker than yogurt or something like that. Just enough to stay more or less put on inclined surfaces but still easily spreadable. I will get the name for you when I get home from work.

I have also tied the straight resin idea and it is a bear to sand. It will stick but sucks to sand for some reason.

So...... whatcha building??? ;D

Cheers
Don

[CARBUILDER Senior Member Find all posts View Profile Close CARBUILDER]

Re: What is the best method to smooth rough fiberglass

Sand down with around 80 grit and use a Resurface Polyester primer like U-Pol Reface Polyester, Evercoat Feather fill, etc. You will also need a large tip to spray it since it is so thick. Don't think about thinning bondo. It is full of tallac and it will soak up moisture like a sponge and the paint will fall off.

I have heard of people rough sanding and then spraying with a sandable gelcoat as a self leveling filler. Have not tried it myself yet so can not say for sure it works for sure.

[bartman Senior Member Find all posts View Profile Close bartman]

Re: What is the best method to smooth rough fiberglass

Sand down with around 80 grit and use a Resurface Polyester primer like U-Pol Reface Polyester, Evercoat Feather fill, etc. You will also need a large tip to spray it since it is so thick. Don't think about thinning bondo. It is full of tallac and it will soak up moisture like a sponge and the paint will fall off.

I have heard of people rough sanding and then spraying with a sandable gelcoat as a self leveling filler. Have not tried it myself yet so can not say for sure it works for sure.

Absolutely to both of these. U-Pol Reface is my favorite for the job, Featherfill is good, Duratec is another excellent product. And Carbuilder is correct when I'm building a plug and one of the above mentioned products is not building enough or I need 4-5 gals to get there, then a primer sanding gel coat thru a dump gun is the way to go to keep from breaking the bank. U-Pol Reface for example is $60/liter, priming gel coat is less than $200/5 gal pail. But you have to understand the difference. The first set of products is made to be applied as a primer, they have oxidizers and other things in them to make them cure as a top coat. The sanding/primer gel does not and once dry it will still be "tacky" on top (waiting on the what would be the fiberglass layer if you were laying up in a mold). So, once cured you will have to sand off that top layer before is will begin to sand and shape like you want it to. All my opinions of course.

[CARBUILDER Senior Member Find all posts View Profile Close CARBUILDER]

Re: What is the best method to smooth rough fiberglass

[quote=bartman ]

Sand down with around 80 grit and use a Resurface Polyester primer like U-Pol Reface Polyester, Evercoat Feather fill, etc. You will also need a large tip to spray it since it is so thick. Don't think about thinning bondo. It is full of tallac and it will soak up moisture like a sponge and the paint will fall off.

I have heard of people rough sanding and then spraying with a sandable gelcoat as a self leveling filler. Have not tried it myself yet so can not say for sure it works for sure.

I have not done this in awhile so the ingredients might be off but try some gelcoat, catalyst, air dry and a filler (powdered or shredded mat fibers). It made a really good hole filler, was sandable and easy to level but you had to work fast. Better than bondo.

[Daleford Senior Member Find all posts View Profile Close Daleford]

Re: What is the best method to smooth rough fiberglass

Thanks everybody for your input. I will investigate and experiment.
Someone asked what I am building. A 6 foot tall giant football helmet for a local collage. I am mounting it on a golf cart and they are using it in parades to promote their team.

[496fe Senior Member Find all posts View Profile Close 496fe]

Re: What is the best method to smooth rough fiberglass

What do you guys think of making your own liteweight resin sanding filler by mixing up some resin and something like Cabosil or other glass-bubble thickener?

[bartman Senior Member Find all posts View Profile Close bartman]

Re: What is the best method to smooth rough fiberglass

What do you guys think of making your own liteweight resin sanding filler by mixing up some resin and something like Cabosil or other glass-bubble thickener?

This is only my opinion, that's it. I've been at this for a long time though. The commercially available stuff like "vette bond" for bonding 'glass to 'glass or fusor for 'glass to steel. And then for priming and smoothing, the other products like U-Pol, or featherfill. Work better than homemade or shop made stuff. They have just the right blend of resins and fillers to work optimally. If you use resin and add cabosil until it is smooth, once cured, it will sand like concrete and defeats the purpose of smoothing. If it shrinkis too much over time it will crack. The commercially available products have many types of fillers in the them and each have thier own recipe of Talc, Titanium Dioxide, microshperes and others some even have wood flour. I guess my point is in my opinion it just isn't worth it. I have my own concoction for a couple things like where I have an area that is too thin and the contour it just too difficult to add more laminations or too difficult to get to then I make some stuff with epoxy, Cabosil, and 1/4 chopped strand to make a paste to back up the panel. The only reason is I have not found another product to fill the need.

[Daleford Senior Member Find all posts View Profile Close Daleford]

Re: What is the best method to smooth rough fiberglass

OK Here is what I did. I didn't have access to any hi teck priming gelcoats or the like. I ground the fiberglass as smooth as I dared and skimmed on a layer of good quality auto body filler. I made a curved "shoe" for the air board sander that matched the curvature of the helmet. I used 40 grit to smooth everything down. This showed a few valleys so I filled those. I am in the process now of sanding everything with 80 grit and finer. I will follow with a few coats of high build 2 part sprayable autobody putty. Done it this way before - should be fine.
Thanks for all input.