What Grit Size - Cured Polyester Resin?

[Wotname]

What is the appropriate grit size to use when abrading a cured polyester resin surface to get good bonding when adding an additional fibreglass layer (using polyester resin), assuming no additional shaping is required?

I have always used 60 or 80 but I dunno why I have used that, maybe just habit!

[s/v Jedi]

Originally Posted by Wotname

What is the appropriate grit size to use when abrading a cured polyester resin surface to get good bonding when adding an additional fibreglass layer (using polyester resin), assuming no additional shaping is required?

I have always used 60 or 80 but I dunno why I have used that, maybe just habit!

Use 60 grit and a fiberglass like 1708 if it needs to be strong or just mat if it’s just to refinish a surface. Don’t use a woven cloth.

[Chotu]

I have to agree with between 60 and 80.

It provides a nice keyed surface with plenty of depth to the grooves. This was used for all bonding throughout my entire boat.

[Wotname]

Originally Posted by s/v Jedi

Use 60 grit and a fiberglass like 1708 if it needs to be strong or just mat if it’s just to refinish a surface. Don’t use a woven cloth.

Thanks.
As a side note, what is the issue with using woven cloth?

[s/v Jedi]

Originally Posted by Wotname

Thanks.
As a side note, what is the issue with using woven cloth?

Adhesion to the roughed up surface. For laminates it used to be mat first, followed by roving or cloth and mat between each layer of woven fabric after that, as well as the last layer.

Now you can just use 1708 which has mat underneath and it is non-woven (the whole thing is stitched).

Especially with using polyester, which has less bonding strength over cured surface than epoxy, it helps to use what adheres best.

[FPNC]

40 grit. Do not 'clean' the surface with solvent. This adversely effects bonding. Only brush or blow it off before applying the next layer.

[Benz]

60-80 is fine, but when I want bombproof adhesion of more laminate to an already cured surface, I like to hit it with 24 or 36 grit on a grinder.
Grinding, I find, is faster than sanding.

[Matt Johnson]

36/40 on a RO in place of using a grinder is the typical suggestion I've read in the spec sheets for polyester.

A large weave fiberglass requires CSM due to polyester's low gap filling quality. The thin stands of CSM fill the voids in between the weaves. I was advised that 12oz (450gm) or less fabric weight works well without mat. It's what you'll see used on a lot of performance boats.

[Wotname]

Originally Posted by FPNC

40 grit. Do not 'clean' the surface with solvent. This adversely effects bonding. Only brush or blow it off before applying the next layer.

Interesting. Any idea why this is so?

I prefer to vacuum the surface after any sanding first before blowing it. I sorta thought a final acetone clean was not always necessary but I hadn't considered it may be detrimental.

Always assumed any waxiness had to be removed before any grinding or sanding.

[Matt Johnson]

I use styrene to clean before bonding.

[s/v Jedi]

Originally Posted by Wotname

Interesting. Any idea why this is so?

I prefer to vacuum the surface after any sanding first before blowing it. I sorta thought a final acetone clean was not always necessary but I hadn't considered it may be detrimental.

Always assumed any waxiness had to be removed before any grinding or sanding.

Yes, clean before grinding/sanding, vacuum (or blow if you’re masochist) then layup. The very rough surface is fresh and wiping will only contaminate it.

If you don’t clean before sanding/grinding, the surface wax and oil gets on the abrasive and you work it into the freshly exposed substrate, ruining the job.

[Wotname]

^^
Got it!

[Wotname]

Thanks all.

While I am at it, what is everyone's goto favourite method of cleaning off the surface wax (poly resins)?

[Jammer]

Detergent, water, and a green scrubbie.

[s/v Jedi]

Originally Posted by Wotname

Thanks all.

While I am at it, what is everyone's goto favourite method of cleaning off the surface wax (poly resins)?

First a scrub with Dawn, rinse, dry, then a wipe with denatured alcohol. Now grind and lay up fiberglass.

For paint it is the same except you sand first. Just before applying paint I use a tack cloth as well.