Sand paper marks

[RumrationParticipant @rumration]

Your advice please. I'm rubbing down an ABS plastic hull, along the joints that were glued with Stabilit. My query is do I have to remove all traces of the scratch marks, or just minimise them and then cover them over with my undercoat and then rub down the paint. I realise that I made need to apply several coats first.

Thanks in advance,

Cheers Glyn.

[RumrationParticipant @rumration]

Removing sanding scratches

[Dodgy Geezer 1Participant @dodgygeezer1]

You will find that, if you use different grades of sandpaper, the marks left by the coarse cutting paper you used initially are flattened and disappear as you use finer and finer sandpaper/glasspaper grades. Eventually they will be smoothed right into the surface, and covered by your undercoats…

[RumrationParticipant @rumration]

Ok, Thank you that's what I'll do then.

Much obliged, Glyn.

[BanjomanParticipant @banjoman]

Dodgy Geezer is quite right that using finer and finer grades of paper (and once you go fine enough, switching from dry to wet paper is also helpful) is likely to remove most of your scratch marks to the point were they will disappear completely below your undercoats.

However, to answer your specific question, in my opinion it is not a very good idea to use normal paint as filler. In a perfect world (ha!), any surface to be painted should be prepared to the standard of smoothness you want the top coat to have, and only then should any paint be applied. Unless you put the paint on rather too thickly, it has a wondrous capacity for highlighting rather than hiding any underlying imperfections. Also, the sanding quailities of paints can vary quite a bit

If you find that just sanding is tricky or insufficient for whatever reason, it is better to use some kind of filler to help smooth things out before applying paint. Depending on what sort of imperfections you are dealing with, and what the resulting surface must be able to cope with, there are different options.

Personally I have at some time or another used the following four types of filler on boats' hulls:

U-Pol High#5 High Build Filler (**LINK**)

Polyfilla standard (wood) filler

David's Isopon P38 Super Body Filler (**LINK**)

Milliput Standard Yellow-Grey two-part epoxy putty (**LINK**).

Actually, I have usually used several or even all of them on the same hull, to do different jobs.

These all have their stronger and weaker points:

U-Pol High#5 is applied from a rattle can, and is essentially a sandable, high-build primer paint. It is solvent based, and very smelly; due to the solvents, I have also found it prone to pinholing unless applied with care. It will not fill any very deep imperfections, but is great for a slightly scratched surface. It is very agreeable to sand (wet paper will give the best results), and the end result can easily outdo most babies' bottoms for smoothness.

Polyfilla standard filler, I have found very useful for smoothing out slightly deeper dings and scratches in spots were the resulting surface doesn't need to be rock hard. It is very easy to apply, and easy enough to sand back.

If, on the other hand, rock hard and absolutely water proof are requirements, P38 is a great product! It stinks to high heaven, and can be a tad messy to apply. Also, it can be slightly tricky to mix filler and hardener to just the right proportions, as only a small quantity of hardener is needed. If you add too much, you will only have a couple of minutes of working time; if you add too little, it will take ages to or even never even at all go off fully. Once hardened it is very nice to sand, though, and becomes very strong.

Milliput, finally, is very useful where you need plenty of working time, as it remains workable for at least half an hour and takes up to four or five hours to fully go off. It is great for smaller filling jobs, including those where the filler needs to be shaped or sculpted to fit or create a shape. Once hardened, it can be drilled, sanded, gouged, turned … Best of all, though, is that a liberal use of water (on a tool or just on a finger tip) when applying it allows one to pre-smooth it to a point where hardly any sanding is needed afterwards.

One very important thing, no matter what combination of fillers and/or paints you use, is to always do a test piece first on some scrap material with the complete set of products in the order you intend to use them. Many — even most — products work very well together, but should there be an incompatibility somewhere in the chain, it is so much better to discover that elsewhere than on your model!

Mattias

 

Edited By Banjoman on 25/01/2017 08:29:59

[Paul TParticipant @pault84577]

Dont use sandpaper on a plastic hull as it creates more problems than it cures, use a decent nail file lightly on the odd rough bit and then go for 1200grit wet and dry followed by 2400grit wet and dry this will create a super smooth scratch free finish

Edited By Paul T on 25/01/2017 13:06:52

[RumrationParticipant @rumration]

Well thanks so much banjo man, such a detailed reply. Fantastic. I have printed it and stuck it over my workbench.

And thanks Paul, I didn't mean sand paper, I only have wet and dry. For info, what is the problem with sand paper, does it leave scratches and debris behind?

[Paul TParticipant @pault84577]

Hello Glyn

Sandpaper (glasspaper) is designed to remove material such as wood or filler as quickly as possible and has an obligingly rough surface made from different grades of crushed glass.

Having such an abrasive surface even the fine grades will leave scratches, this doesn't really matter on timber as the scratches become 'absorbed' within in grain but they stand out a mile on rigid surfaces like plastic.

Hope this helps explain

Paul

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