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DanS911's Avatar
 
Join Date: Aug 2003
Location: South Florida
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How much sanding is enough

Ok so five or ten years ago I bought a bunch of really nice Getty Design F/G parts.

The front and rear bumpers have been on the car for a few years and the fenders up in the attic.

I had this black and blue thing going on so was fine with out sanding or painting the bumpers.
Now preping for some paint work and wondering how much sanding is really necessary. The slightly uppity website says sand the crap out of them and if it takes more than an hour you aren't really trying!
Well i am really trying. I have a 7in sander with a 80grid. and a hand held orbital with the same.
Am I really trying to sand all the way to the glass!?
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Dan
'86 911 Targa Driver
'76 911 Targa 3.0 Track Toy
46mm PMOs, 10.5/1 J&E,Web Cams, Wide Body fenders, 23mm and 30mm Hollow T-Bars, 930 Sway bars, Bilstein Sport Shocks, Plastic Bushings (too damn squeeky)
Old 12-26-2013, 04:26 AM
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dkbautosports.com
 
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ok 1st
the parts should have a gelcoat on them .
so why are you trying to sand it off down to the raw fiberglass ?
who ever told you that you have to spend at least an hour on each part is a total moron !

if your looking to just top coat the gelcoat all you have to do is sand the parts with 220 or 320 grit seal and paint .
now if there are things you want to fix before you do any painting like seams were two or more pieces were bonded together on the part or it's got pin holes in the gelcoat or the part is wavy and you want to block out the waves then you do your sanding with 80 grit and do your body work to fix the areas you need to . after your finished with the body work you do your final sand before priming with 180 grit . then mix your filler primer and prime the parts .
let the primer dry and do your prep sand for top coating with 400 to 500 wet or 320 to 400dry . then your ready for your top coating .

all you have to do for any product to stick is just sand every part of the pieces . if you don't sand a area the primer or top coats will not stick to the parts .
so you don't miss an area you should spray a guide coat on the parts .
that will show you were you have to sand and any areas you may have missed when your sanding . it will also show any pin holes you may not have seen along with other imperfections .

at most all you need is a orbital sander and some hand sanding . no 7" sander ?
if you have to do some fiberglass and resin work that is the only time you have to strip off the gelcoat down to raw glass .
Old 12-26-2013, 05:26 AM
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^^This^^

Other than fixing what doesn't fit (front bumper too wide, tail doesn't fit in opening, etc) fiberglass parts are easy. Watch out for pinholes (fix them now or they pop through the paint), wavy panels must be straightened (this is the number one reason these parts look like fiberglass), and seams, cracks, and holes filled. 80 grit for filler, 180 grit for 3 to 4 coats of filler primer, guide coat, block sand with 400 wet until smooth (guide coat removed), and top coat and clear (if required). If you remove the gel coat, your primer and top coat will lift fiberglass fibers and look bad. Gel coat is there for a reason, but often has voids and pinholes, so careful prep is mandatory.

As an example, my rear RSR bumper took an hour to scuff up with 180 grit, two hours to fill voids, pinholes, and waves and sand smooth, 30 minutes for 4 coats of filler primer (flash time), two hours wet sanding guide coat with 400, and 30 minutes to top coat. Of course there were several days of cure time, work, other house crap in between each step, but actual time totaled about 6 hours of true work.
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Old 12-26-2013, 06:10 AM
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now I have been working with fiberglass and carbon fiber for years started with fiberglass 40 years ago . it's how I made part of my living as I owned a race shop for most of my life . so we/I have built more than I can remember full bodys , panels , parts and any thing else you can think of out of fiberglass let alone the amount of repairs that we/I have done .

so priming over even raw fiberglass and doing any plastic work will in no way lift the glass mat or cloth if done the right way with the right products .

many race cars and even some high end sports car do not gelcoat there fiberglass parts .
gelcoat is not needed but for a couple of reasons and non are for any thing important .

if your looking to block a FG part good and straight you don't really want to use a primer you use what is called sprayfiller . most are of polyester based to match the (in most cases) the resin base of the gelcoating or even the fiberglass resin .
there are other fiberglass resin but they are not used all that often on parts for cars as the resin base is not needed because cars unlike boats don't see salt water .

you should not be spraying 3 to 4 coats of a filler primer and blocking then spraying another 3 to 4 coats blocking and so on with a filler primer . the reason is stacking so much filer primer the primer will delam over time .
the reason for this is your filler primers have what is called a MILL thickness rule .
were a spray filler has a much higher mill thickness rule .
so you should if your looking to block a part nice and straight use a sprayfiller if your thinking you will need more than the 1 time priming of 3 to 4 coats of a filler primer .
Old 12-26-2013, 07:09 AM
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Great info.

What do you guys do about hairline cracks in the fiber glass/stress cracks???

Many sites recommend refiberglassing, but that seems like overkill.

Anyone try seeping superglue into the cracks and following up with epoxy primer? Or, just adding a flex agent to the paint?

Any real world easy ways to deal with that???

Bo
Old 12-26-2013, 10:47 AM
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it depends on from were the cracks are coming from .
if the cracks start in the fiberglass cloth or mat and that is what cracked the gelcoat then you have to remove the gelcoat all the way down to the raw glass and repair the fiberglass .
if your cracks are only in the gelcoat you just have to remove the gelcoat depending on how thick the gelcoat is you may or may not be able to feather out the gelcoat and just spray fill it .
there are different way to go about replacing the gelcoating .
you can replace it with more gelcoat or use a body filler and just use a filler primer over it or you can use a spray filler doing it in multiple applications as you would do with a gelcoat .
again all that depends on the gelcoat that's on the part and just how thick the gelcoat is .
if it's known as light weight fiberglass laminate there will be no gelcoating at all . so then you would want to just use a filler primer to keep the weight as low as possible .

so there is no real ez way to get rid of cracks .
you could use what is called a gelcoat paste . you V-out the cracks and with a body filler spreader fill in the cracks . let dry then block the area and repeat as needed until the cracks are filled .
this way how ever works great on things like boats but not so good on cars panels .
the reason is on a boat your mixing the color of the boat into the gelcoat putty and not top coating it with auto paint . with auto paint you will end up seeing a ghost of the cracks thru the finish .
Old 12-26-2013, 11:44 AM
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Thanks for all the info,
I would have been sanding for days!
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Dan
'86 911 Targa Driver
'76 911 Targa 3.0 Track Toy
46mm PMOs, 10.5/1 J&E,Web Cams, Wide Body fenders, 23mm and 30mm Hollow T-Bars, 930 Sway bars, Bilstein Sport Shocks, Plastic Bushings (too damn squeeky)
Old 12-27-2013, 06:34 PM
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Quote:
Originally Posted by 962porsche View Post
it depends on from were the cracks are coming from .
if the cracks start in the fiberglass cloth or mat and that is what cracked the gelcoat then you have to remove the gelcoat all the way down to the raw glass and repair the fiberglass .
if your cracks are only in the gelcoat you just have to remove the gelcoat depending on how thick the gelcoat is you may or may not be able to feather out the gelcoat and just spray fill it .
there are different way to go about replacing the gelcoating .
you can replace it with more gelcoat or use a body filler and just use a filler primer over it or you can use a spray filler doing it in multiple applications as you would do with a gelcoat .
again all that depends on the gelcoat that's on the part and just how thick the gelcoat is .
if it's known as light weight fiberglass laminate there will be no gelcoating at all . so then you would want to just use a filler primer to keep the weight as low as possible .



so there is no real ez way to get rid of cracks .
you could use what is called a gelcoat paste . you V-out the cracks and with a body filler spreader fill in the cracks . let dry then block the area and repeat as needed until the cracks are filled .
this way how ever works great on things like boats but not so good on cars panels .
the reason is on a boat your mixing the color of the boat into the gelcoat putty and not top coating it with auto paint . with auto paint you will end up seeing a ghost of the cracks thru the finish .
Thanks!

Bo
Old 12-27-2013, 07:15 PM
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