Body filler vs topcoat

Anything goes in the world of fiberglass and plastic



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PostPosted: Sat Jun 18, 2022 12:51 pm
Hi , im just wondering so if my intention is to paint it what is really the difference between topcoat and body filler( primer and paint after ) I see every auto body part being sold with gelcoat/topcoat but ive been told i could get away with body filler and then painting .

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PostPosted: Sat Jun 18, 2022 10:40 pm
Ok, lets get some terminology straight, first.

Body Filler
Usually a polyester resin based product that is mixed with a hardener and applied to bare metal, treated or epoxied surface to take up surface variations (i.e. dents) and then sanded smooth. Comes in various grades right through to very fine glaze.

Can also come as a spray-on coating that is, again, sanded flat and smooth. Needs a primer applied before topcoating.

Primer
Intended to be the layer between the substrate and the topcoat because it will stick to the appropriate substrate as well as providing an excellent base on which a topcoat can be applied. Primers can be of different chemical compositions, depending on need but the most common are epoxy, urethane and acrylic.

Filler Primer
Also known as hifill/highfill and various other trade names. A much thicker primer that not only provides the "glue" between base and topcoats but can also can fill small imperfections in the surface and then be sanded smooth and flat.

Topcoat
Lots of different kinds ranging from a single application gloss finish through to multiple application/layer finishes with clear coats that can be gloss, satin or matte, but essentiall what is on the top and what you see and feel. Provides protection for whatever is underneath.

Gelcoat
A special type of topcoat which is not really a paint but an epoxy or polyester that's used usually only with fibreglass. It protects the fibreglass underneath from sun, salt and various other things.

That's pretty brief - the full course goes for about 4 years and even then there's no guarantee that you'll know it all.

Now, to your question.

It all depends on the condition of your substrate or whatever metal or plastic is underneath. If it's like new and undamaged then often just a single coat of primer, followed by topcoat(s) may be all that's necessary.

If it's been damaged and has been repaired then depends on the quality of the repair. Really good panel beaters can get a panel back to "like new" smoothness and shape but those with lesser talents may need to rely on fillers to cover their shortcomings. What kind of filler depends on the level of "shortcomings".

Either way it's always preferable to get any substrate as near as is possible/practicable to perfect. The thicker the layer(s) above it the greater the likelihood that the repair will fail.

Why don't you take some pictures of what you're planning to paint and let's offer you some recommendations.
Chris



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PostPosted: Sun Jun 19, 2022 11:57 pm
NFT5 wrote:Ok, lets get some terminology straight, first.

Body Filler
Usually a polyester resin based product that is mixed with a hardener and applied to bare metal, treated or epoxied surface to take up surface variations (i.e. dents) and then sanded smooth. Comes in various grades right through to very fine glaze.

Can also come as a spray-on coating that is, again, sanded flat and smooth. Needs a primer applied before topcoating.

Primer
Intended to be the layer between the substrate and the topcoat because it will stick to the appropriate substrate as well as providing an excellent base on which a topcoat can be applied. Primers can be of different chemical compositions, depending on need but the most common are epoxy, urethane and acrylic.

Filler Primer
Also known as hifill/highfill and various other trade names. A much thicker primer that not only provides the "glue" between base and topcoats but can also can fill small imperfections in the surface and then be sanded smooth and flat.

Topcoat
Lots of different kinds ranging from a single application gloss finish through to multiple application/layer finishes with clear coats that can be gloss, satin or matte, but essentiall what is on the top and what you see and feel. Provides protection for whatever is underneath.

Gelcoat
A special type of topcoat which is not really a paint but an epoxy or polyester that's used usually only with fibreglass. It protects the fibreglass underneath from sun, salt and various other things.

That's pretty brief - the full course goes for about 4 years and even then there's no guarantee that you'll know it all.

Now, to your question.

It all depends on the condition of your substrate or whatever metal or plastic is underneath. If it's like new and undamaged then often just a single coat of primer, followed by topcoat(s) may be all that's necessary.

If it's been damaged and has been repaired then depends on the quality of the repair. Really good panel beaters can get a panel back to "like new" smoothness and shape but those with lesser talents may need to rely on fillers to cover their shortcomings. What kind of filler depends on the level of "shortcomings".

Either way it's always preferable to get any substrate as near as is possible/practicable to perfect. The thicker the layer(s) above it the greater the likelihood that the repair will fail.

Why don't you take some pictures of what you're planning to paint and let's offer you some recommendations.


Thank you very much, well im starting from fresh a spoiler with foam , so after fiberglass layers im just wondering, what if its slightly not even , what if 1 corner i would like higher etc which body filler would sort it

But same time ive read that body filler does not give you any strenght or structure while gelcoat/topcoat does .

Also would it be smart to do topcoat and then spray paint my own colour anyway ?

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PostPosted: Mon Jun 20, 2022 12:45 am
Oh, so this is going to be a serious learning exercise.

josh231 wrote: ive read that body filler does not give you any strenght or structure while gelcoat/topcoat does .


If you have a corner or something that needs a little more then the best way is to lay a little mat and resin. That is the way to get maximum strength.

Alternately, you could use glass reinforced filler, which is also quite strong,

The third way, for smaller adjustments in shape or for filling sanding scratches would be just normal filler. Not so strong by itself, but over a substrate like fibreglass and with thickness limited to 5-8mm, still strong enough.

Gelcoat is a topcoat. When a boat, for example, is laid up, the first thing that gets sprayed into the mould is gelcoat. When the boat (or anything else) comes out then the gelcoat is the external layer. For most production type boats no paint is used - just the gelcoat, which will take many years of exposure. High end boats and things like car parts are then painted, over the gelcoat, to achieve colours or finishes that aren't possible with gelcoat alone.

josh231 wrote:Also would it be smart to do topcoat and then spray paint my own colour anyway ?


In your case you're building up from foam and, unless you're going to use what you make as a plug to make a mould for future production, then gelcoat is entirely unnecessary. Build up the shape you want, smooth it using a filler for any lows or voids and then prime and paint it.
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PostPosted: Mon Jun 20, 2022 1:51 am
NFT5 wrote:Oh, so this is going to be a serious learning exercise.

josh231 wrote: ive read that body filler does not give you any strenght or structure while gelcoat/topcoat does .


If you have a corner or something that needs a little more then the best way is to lay a little mat and resin. That is the way to get maximum strength.

Alternately, you could use glass reinforced filler, which is also quite strong,

The third way, for smaller adjustments in shape or for filling sanding scratches would be just normal filler. Not so strong by itself, but over a substrate like fibreglass and with thickness limited to 5-8mm, still strong enough.

Gelcoat is a topcoat. When a boat, for example, is laid up, the first thing that gets sprayed into the mould is gelcoat. When the boat (or anything else) comes out then the gelcoat is the external layer. For most production type boats no paint is used - just the gelcoat, which will take many years of exposure. High end boats and things like car parts are then painted, over the gelcoat, to achieve colours or finishes that aren't possible with gelcoat alone.

josh231 wrote:Also would it be smart to do topcoat and then spray paint my own colour anyway ?


In your case you're building up from foam and, unless you're going to use what you make as a plug to make a mould for future production, then gelcoat is entirely unnecessary. Build up the shape you want, smooth it using a filler for any lows or voids and then prime and paint it.


thank you this is first time im getting such a detailed explanation, just to ask to if i dont use gelcoat/topcoat and if i dont need body filler through the entire thing ( just few spots ) i would need something anyway otherwise the fiberglass matting would be exposed , no ?

So once again i would either need to body filler all over to hide matting or topcoat/gelcoat or im wrong

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PostPosted: Mon Jun 20, 2022 3:50 am
The way it seems that you're going to do it, you'll sand the resin and mat until it's the shape you want. That will leave some of the glass exposed. I would then put a light coat of resin over that to seal it up, since water can travel along the glass strands.

Once sealed, a light, skim coat of filler to take out any texture and then sanded smooth. But even at this stage you'll still be shaping a bit so sandpaper might be quite coarse. You could skim with a fine filler or glaze or you could use a couple of coats of a primer/filler before the final sanding of P600-P800 needed for the paint topcoat(s).

There is no hard and fast rule. Each layer goes on according to the finish of the previous layer, but, in general, no, you don't need to cover the whole thing in filler if the shape is good. You should, however, prime it all before topcoats.
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PostPosted: Mon Jun 20, 2022 5:29 am
NFT5 wrote:The way it seems that you're going to do it, you'll sand the resin and mat until it's the shape you want. That will leave some of the glass exposed. I would then put a light coat of resin over that to seal it up, since water can travel along the glass strands.

Once sealed, a light, skim coat of filler to take out any texture and then sanded smooth. But even at this stage you'll still be shaping a bit so sandpaper might be quite coarse. You could skim with a fine filler or glaze or you could use a couple of coats of a primer/filler before the final sanding of P600-P800 needed for the paint topcoat(s).

There is no hard and fast rule. Each layer goes on according to the finish of the previous layer, but, in general, no, you don't need to cover the whole thing in filler if the shape is good. You should, however, prime it all before topcoats.

Would i not be better off with spray painting as i dont have a way to spray paint top coat and wouldnt it look a bit odd with brush marks from top coat ? Also if i would not paint and so top coat would be last layer , what happens in the future if i want to repaint it would all the topcoat needs to be removed?

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PostPosted: Mon Jun 20, 2022 8:02 am
Go back to my first post. There is no such thing as "topcoat". There is paint or there is gelcoat. Either one can be sprayed on (with the right equipment) and either of these can be the top coats. Or you can paint over gelcoat. In your case there's no need for gelcoat. Filler is cheaper, stronger and can be built up higher.

So, first you would lay 2-3 layers of fibreglass and resin over your foam. If you haven't fibreglassed before that's going to end up a pretty rough finish. Then sand it back to about what you want before a thin coat of resin to seal maybe with a little mat to fill the big holes. Follow that by applying a filler. Sand that back smooth (may need to do a couple of times), possibly finishing with a fine filler or glaze. Sand that back. At this point it doesn't matter if parts of the glass/resin are visible. All that matters is that the surface is smooth and flat. Then prime, sand again and paint.
Chris

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