Is poly abrasive safe on 'glass?

[Patricia Brown]

Hello,

I am preparing the underside of my '72 Corvette for gel coat, which requires an 80 grit surface. I am getting discouraged with the tedium of hand sanding and have a LONG way to go.

So, I am highly tempted to try blasting it. I have read that extreme caution is necessary when blasting 'glass so as not to harm it. It appears that "poly" abrasive from the Eastwood catalog may be my gentlest option. Has anyone tried this, and is it safe if I keep the pressure low and watch things? I sure don't want to "thin" the fiberglass or burn a hole through it, yet the description for poly claims it's safe for fiberglass. (I am aware of soda blasting, but I would like to do this project myself, and I don't want to invest in a soda blaster.)

I'm assuming I should start with the gentler grit, 20/30 poly, as opposed to the available 12/16 grit, but again, I'd appreciate any advice.

And, finally, the gel coat wants an 80 grit surface to cling to, and I'm wondering if anyone knows whether either of the two available poly abrasives would yield such a surface.

Thank you,
Patricia

[Michael L.]

Re: Is poly abrasive safe on 'glass?

Patricia, some of the plastic abrasives are OK for fiberglass. I'm not familiar with Eastwoods, but I'd imagine thier tech desk could answer that question. My only question would be: how are you going to do this at a reasonable cost without being able to recycle the plastic blast media? You may want to consider costing out that method vs. paying someone to come to the house and soda blast, if that service is available in your area. I know of a person in Reno, where I live who comes to your project. There is a big mess involved with soda blasting as well.

If you can contain the plastic blast media and filter, that could work for you at a reasonable cost and save time. I know that6 doing it yourself is very satisfying, I have struggled with these options as well.

Mike

[Patricia Brown]

Re: Is poly abrasive safe on 'glass?

Mike, thank you for the reminder of expense due to media being lost. That is definitely a concern, especially when this stuff is $100 for only 50 lb! I'm wondering how much I will need. I'm mainly just looking at the bottom of the floor plus the gas tank area. I'm hoping 50 lb. would do it.

I will have to find out if there is a visiting soda blast service in the area, but I kind of doubt it.

Well, so long as I'm not being too crazy trying poly on fiberglass I may just give it a whirl... carefully. If anyone has done this and can recommend a particular brand and/or grit size, I'd sure appreciate it! The only stuff I've seen is Eastwood's, but perhaps there's something more appropriate out there.

Thanks,
Patricia

[Phil P.]

soda blasting you wash away residue w/water *NM*

[Kent K.]

Re: Is poly abrasive safe on 'glass?

Patricia,

I've been reading your posts and responses and, for the life of me, can't understand why you want to gel coat the underside of a Corvette. Most, if not all of them were just plain fiberglass without paint other than overspray and sometimes some blackout. Seems like a lot of unnecessary work.

Regards, Kent #6201

[Chuck S.]

Re: Is poly abrasive safe on 'glass?

"...thank you for the reminder of expense due to media being lost. That is definitely a concern, especially when this stuff is $100 for only 50 lb! I'm wondering how much I will need. I'm mainly just looking at the bottom of the floor plus the gas tank area..."

Patricia, is there a reason for your wanting to gelcoat the underbody? I thought gelcoat was to repair minor surface roughness from exposed fibers, and to seal the fiberglass to insure no trapped solvents causing problems with the repaint. The underbody will ususally clean up to slick, like new fiberglass, and that's the way it should be. Gelcoat may be the answer if your underbody is in really rough condition.

No doubt about it...sanding stinks. A painting professional once said, "Where ever possible use a machine to lessen the work"...Get yourself a random orbital palm sander to use on the big flat areas (tops, doors, hood/nose, deck, and most of the fenders). Hand sand only the curves with a soft block or pad. Unfortunately, there are a lot of curves on a 72, but the sander will cut the work considerably.

I have a DeWalt palm sander that accepts the "hook-it" discs (like velcro; allows quick disc changes) with a dust collector. A palm sander will work well on grits up to about 400 if you can sand dry; for wet sanding with finer grits, you will need a specialized wet sander like a "waterbug". At very fine grits, you can no longer get the "hook-it" discs...you have to use the self-adhesive discs (sticky backed; slightly more trouble to change).

[Patricia Brown]

Re: Is poly abrasive safe on 'glass?

Hi Kent and Chuck,

The reason I am wanting to gel coat the underside of my '72 is to seal over the many repairs I am having to make, and, mainly, to guard against moisture working its way through the fiberglass panel to cause problems with the paint. Unfortunately, I am having to install several aftermarket panels, and even though I will go with the best quality, press-molded parts from what I hear they are still not as dense as the factory 'glass and are not as resistant to moisture working through them. I plan to drive the car, and I live in a very humid climate, so moisture is a concern to me. From things I've heard it may be wise to have the panels gel coated on the backsides as well as the tops.

Yes, I have a palm sander which does a beautiful job, only it seems it doesn't fit hardly anywhere on the underside of the car-too many nooks and crannies! I've got the wheel wells all done and most of the gas tank area, but the whole floor has yet to be done... I think when I finally get to sanding the *topside* of the car, in preparation for paint, it will seem very easy compared.

[Chuck S.]

Re: Is poly abrasive safe on 'glass?

For the "nooks and crannies", you might want to look for a small specialized sander that uses sandpaper shapes. Mikata makes one that uses little triangle shaped sheets; it will sand deep in corners that your finger can't even reach. Dremel also makes a small reciprocal sander (my favorite) with changeable profiles (small radius, medium radius, large radius, flat triangular sheet, etc.).

One of the reasons I prefer RO machine sanders is that you are less likely to sand into the surface very faint wave patterns by inadvertantly using the exactly same pattern every time. For example, if you begin at the door gap, you may methodically sand one pass from top to bottom of the door, the length of an easy hand stroke, say 6"-8".

Once you get to the bottom of that pass, you may advance to the next pass and overlap the strokes, once again, a stroke of about 6"-8"...where the passes overlap, you have probably removed more material. Do this several times the same way with primer or primer-surfacer, and the cumlative effect is to build in vertical tiger stripes. Varying the overlap of passes is just as important during paint spraying; if you don't change it up, you end up with thick and thin stripes.

For what it's worth, I ain't buying that absorption of atmosphereic moisture into the panels is ruining the paint...IMO that's an excuse by someone covering up solvents coming out of their panels. If absorbed moisture was a problem, you could never paint these cars in humid areas, and we know it's done all the time without problems. That doesn't mean you should be trying to paint when it's raining outside... (I have seen it tried.)