Sanding

hello! I would like to apply epoxy over resin over polyester gelcoat. I have seen several instructions on how I should do it. Degrease first, then sand with 80 grit, then degrease again. Others said to degrease after sanding, and after sanding, just blow or vacuum without degreasing at the end. Which is the best for a strong adhesion? Thanks

 

Your first sentence was a bit confusing, but...

Cleaning the surface of anything you're applying something over prior to sanding is good idea if needed, it's not always required though. And it also depends on the grit you're using, the finer the grit the more likely surface cleaning will help. 400 grit isn't removing much material, so cleaning helps a great deal, 36 grit is removing a good deal of material and flinging it elsewhere, so cleaning doesn't help that much.

It also depends on what surface contaminants you're trying to get rid of. If the item was waxed two years ago there is little, and more likely no wax left on the surface, but if you're redoing an old bilge with decades of gunk built up in it, you may need to clean it several times before you start sanding.

Cleaning after sanding is a different process, but again, isn't always required. For low viscosity paints that will fisheye at just the thought of contamination, it can help a great deal. After grinding a surface with 36 grit you most likely don't need to clean it other than sweeping, wiping and/or vacuuming the surface.

Cleaning with a surface prep product (not acetone) after sanding with fine grit paper is a good idea. Trying to solvent clean a surface after 36 grit is more difficult, you end up smearing dust and contaminants around until you've filled the bottom of the sanding scratches with dust and debris that dries like mud on the bottom of a dry lake bed.

Although cleaning a small area with acetone might work if you do it properly, just wiping some around on the surface doesn't do anything. You need to use two rags, one with wet with acetone and a clean dry one to pick up the still wet acetone from the surface. Since acetone dries so fast you don't end up picking up much gunk off the surface, you just smear it around. Products designed for cleaning don't evaporate as quickly so you have more time to remove them from the surface after they've dissolved and picked up whatever stuff was there.

Epoxy can blush leaving a wax like film on the surface, this needs to be cleaned thoroughly with soap and water to remove it, solvents won't do it. Then the use a coarse grit to prep the surface.

Always use the coarsest grit possible for a better bond, 80 grit will work, but 36 grit is better. This all depends on exactly what you're putting on the surface though, Imron won't look good over 36 grit, but a laminate over 36 grit will bond much better.

 

Hello! Thanks for the advice! I usually clean wax surfaces with a product dedicated to removing wax, then sand with 80 grit, blow well with air, then clean with isopropyl alcohol and the same wax removal product to make sure that no trace of dust or contaminants remains on the surface.

 

No indication of what you’re actually looking to achieve here, but you’ll get a better bond by sanding through the gelcoat until fiber is exposed.

 

Dewax with solvent.

Sand to 40 grit,

Vacuum.

Blow.

Acetone wash.

Blow.

Keep in mind the bond strength can be no greater than the gc/poly bonds.

 

Not an insult, but what does the acetone wash actually do?
And blowing it off is risky.

 

It depend of dust air contamination .

 

The solvent wash is just to remove any grease contaminant.

The last blow is not blowing acetone or solvent, but any dust that may settle as Montero mentions.

 

Acetone is good at dissolving some contaminants, but the fast evaporation makes it very difficult to actually remove them from the surface, that's why there are products formulated for this purpose. You end leaving lots of unwanted stuff smeared around on the surface with acetone. Although on a very small area you might be able to correctly use acetone, but it doesn't scale up well.

And blowing off a surface is unreliable because few people have the correct air handling equipment to actually produce dry and oil free air, so again, just more contamination.

 

these are all valid and fair points, but this was my tack for most of my work

I also have no desire to argue with an expert and I consider you one.

 

That's kind of the way most people approach it, it's just sort of what's been accepted as "the way to do it". But none of it really helps, it just adds steps, time, money, and creates a stinky mess.

 

You are undoubtedly an expert in a given production process. This doesn't mean that we are currently creating a new production process.