Best method of sanding uneven surface (grp weave)

[Keith-i]

I’d like to repaint the inside face of my transom which has a textured surface to it as a result of the glassfibre weave. Ordinary methods of sanding pass over the troughs which risks poor adhesion of the paint. Do the collective have any recommendations for a method to clean and key as much of the surface as possible?

 

[V1701]

Fine-ish wire brushing, manual or drill attachment? Then a thick paint like a garage floor paint or bilge paint?

 

[Praxinoscope]

On grp surfaces that you describe, I usually give the surface a good clean with Acetone, plenty of it on a cloth so that it reaches troughs as well as the higher parts, this cleans the surface and also removes any residual grp wax, leaving a surface that paint will adhere to.

 

[Poignard]

I have found scrubbing the surface with sugar soap to be very effective, after picking off any loose paint. It is also very cheap.

 

[Keith-i]

Some interesting ideas thanks. The surface has been previously painted. It’s dirty and stained in various places and I don’t think it was prepped properly before it was last painted. There are some patches where it’s peeling too.

 

[geem]

Some interesting ideas thanks. The surface has been previously painted. It’s dirty and stained in various places and I don’t think it was prepped properly before it was last painted. There are some patches where it’s peeling too.

Use a DA sander with some 80 grit. Sand of everything Including the weave pattern. Just done that on a repair today to my dinghy transom. I did use 1200g/m2 cloth though so I know I have plenty of meat to sand back.
It's now super smooth and ready for paint tomorrow ?

 

[Concerto]

Once you have cleaned the dirty surface, why not flow coat it? This will leave it smooth for the future.

Flow coat is gel coat with wax added so the surface dries hard, not sticky like gel coat as this is part of the bonding to the glassfibre.

 

[penberth3]

Scotchbrite pads? Various grades available.

 

[William_H]

I don't think it practical to sand down the exposed weave to a level surface. You need a combination of fill with paint or flow coat followed by sanding down. Note the flow coat will probably stick ok but is really used on a new GRP lay up so GRP is "green" and flow coat can chemically bond to the GRP. ie made for the job you have in mind but not so good for old GRP. ol'will

 

[Stemar]

My weapon of choice for sanding less than perfectly smooth surfaces is one of these:

Abracs Nylon Brush End 75mm | Toolstation

or this:

Abracs Nylon Filament Wheel Brush 75mm | Toolstation

 

[geem]

I don't think it practical to sand down the exposed weave to a level surface. You need a combination of fill with paint or flow coat followed by sanding down. Note the flow coat will probably stick ok but is really used on a new GRP lay up so GRP is "green" and flow coat can chemically bond to the GRP. ie made for the job you have in mind but not so good for old GRP. ol'will

I did sand the weave out yesterday. Very easy but like I say, I have plenty of material to go at and I was only repairing a small area. The problem with weave is you can't easily remove paint and contamination from the bottom of the weave so paint or flowcoat adhesion is a problem.

 

[Keith-i]

My weapon of choice for sanding less than perfectly smooth surfaces is one of these:

Abracs Nylon Brush End 75mm | Toolstation

I thought about a steel wire cup brush but that nylon version is probably better. Thanks