Paint theory - layer basics

[chamy]

I had a piece of fibreglass bodywork I was going to attack in the next few weeks.

I had planned to repair / fill the area with an epoxy filler,, sand it with something like 240 grit, then coat this area with an epoxy primer, sand it with a 500 grit, , then lay the basecoat down and clear.

Have I got this right or missed any steps out?

I discarded the idea of also using a high build primer after the epoxy primer as i thought it was unnecessary.

 

[the snooper]

500 is too course for base coat id finish with 800 for base
apart from that away you go

 

[steveo3002]

500 is too course for base coat id finish with 800 for base
apart from that away you go

agreed if using wet n dry

dry sanding is roughly double the number so would be okay

 

[chamy]

agreed if using wet n dry

dry sanding is roughly double the number so would be okay

This is coming from the spec sheet which states P400-P500 or P800 (W+D).

I havent spoken to any other 'sprayers', but is laying down base coat on top of epoxy primer a common thing to do?

 

[langy]

This is coming from the spec sheet which states P400-P500 or P800 (W+D).

I havent spoken to any other 'sprayers', but is laying down base coat on top of epoxy primer a common thing to do?

Yes base onto epoxy is fine, but the epoxy won't build like hi build will so getting panels straight will be more difficult.

Basic rule of thumb is final sand 600 for straiight colour and 800 for COB

 

[Chickenjohn]

If you want a really good finish spray some coats of high build primer over the epoxy (2 if 2-pack, 4 if cellulose high build) then dust with a black or dark blue paint, sand with 400/500 grit till all the black paint dots are removed, then spray more coats of primer, dust again and sand with 600 or 800 grit paper. If your top coat spraying technique is good you will now have a glass smooth finish.

 

[chamy]

Thanks guys,

Revisited the spec sheet which exactly says:
"Sandable after 1h IR drying
Dry sand with P400-P500
Wet sand with P800"

Does this mean, dry sand p400-500, OR wet sand p800, or does it mean dry sand p400-500 THEN wet sand p800?


The product is actually labelled as a 'epoxy primer / filler'. Im not looking for a mirror finish, just 'good', I assume this is do-able? The colour is a solid black if that makes a difference.

Lastly, what does COB mean?

 

[MG David]

COB = clear over base

 

[langy]

I think your missing the difference between Epoxy and hi build, they do different jobs, Epoxy is anti corrosive and gives an excellent bond to steel, aluminium, GRP but doesn't build up much whereas Hi Build will build up very well and allow you to straighten out by block sanding to a certain degree but doesn't bond as well as the Epoxy, just different products for different jobs.
Most body shops don't use epoxy as its just another cost to endure and they are not that worried about the job lasting as they probably won't see the car again.
This is the way i do it (others will do it differently) but i'm working on peoples expensive pride & joys so they usually prefer to spend a little more.

Same process for steel, aluminium or GRP

Key the surface with 120 grit
apply 2 coats of epoxy primer
key the epoxy when dry with 120 grit
now do the filler work
recoat with epoxy 2 coats, wait 1 hour and then apply 3 coats of hi build wet on wet.
block sand the hi build to 600 grit dry
then apply 1-2 coats of epoxy tinted to the final topcoat colour and sand wet to 600 for straight paint or 800 for Clear Over Base.

 

[Robotstar5]

Key the surface with 120 grit
apply 2 coats of epoxy primer
key the epoxy when dry with 120 grit
now do the filler work
recoat with epoxy 2 coats, wait 1 hour and then apply 3 coats of hi build wet on wet.
block sand the hi build to 600 grit dry
then apply 1-2 coats of epoxy tinted to the final topcoat colour and sand wet to 600 for straight paint or 800 for Clear Over Base.

Followed by how many coats of topcoat & laquer? I can see why custom cars have 20+ coats on

 

[Chickenjohn]

I agree, its better to prime the panel first then fill as Langy does. In my case, I always etch prime bare metal then fill. I have seen quite a few panels where they filled onto bare metal and invariably, a layer or surface rust has started to form where the filler meets the bare metal. Any breach in the top layers will mean the filler will absorb moisture, let in air and the panel will start to rust, unseen. Probably till it rots completely through!

So best to etch or epoxy prime first , then fill.

 

[Chickenjohn]

BTW, Langy- can I spray epoxy with a normal 3M mask?? It's not 2K is it??

 

[langy]

I agree, its better to prime the panel first then fill as Langy does. In my case, I always etch prime bare metal then fill. I have seen quite a few panels where they filled onto bare metal and invariably, a layer or surface rust has started to form where the filler meets the bare metal. Any breach in the top layers will mean the filler will absorb moisture, let in air and the panel will start to rust, unseen. Probably till it rots completely through!

So best to etch or epoxy prime first , then fill.

John I was always taught not to fill on etch primer ?

 

[langy]

BTW, Langy- can I spray epoxy with a normal 3M mask?? It's not 2K is it??

John I will pm you, yes epoxy is 2k, primer plus hardener mate

 

[langy]

BTW, Langy- can I spray epoxy with a normal 3M mask?? It's not 2K is it??

Followed by how many coats of topcoat & laquer? I can see why custom cars have 20+ coats on

Depends on what topcoat really, straight paint 3 coats, Metalflake, Kandies or graphics etc etc need more

 

[Chickenjohn]

John I was always taught not to fill on etch primer ?

Modern fillers seem to work well over etch primer. Never had a problem yet!

 

[langy]

Modern fillers seem to work well over etch primer. Never had a problem yet!

Interesting, I've never tried it as i always epoxy everything.