Primer selection

Anything goes in the world of fiberglass and plastic



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PostPosted: Mon Apr 27, 2020 12:03 pm
I'm going to be sanding down my polyurethane bumper down to plastic and redoing the bumper that has paint peeling everywhere.
After I sand it I will be applying adhesion promoter. I'm not sure if I should use epoxy primer or 2K primer, The bumper is about 2 years old and I'm sure there's a rock chips and I might have to do some sanding.

But I'm a little bit confused on the epoxy primer. It says it's a primer sealer, It also says it's sandable . does that mean if I mix it with an activator it's only a primer and I have to add reducer for it to become a sealer?

Should I use it as primer? Should I use 2K and use the epoxy as sealer on top of it? Not sure what the next step should be here. I have dry Guide Coat that I'm going to be putting on top of the primer and using that to find any imperfections, I don't know how much sanding is acceptable on the epoxy

I was told by someone to use epoxy as a primer so I went and I already bought it but now someone else is telling me to use 2K if it needs sanding.

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PostPosted: Mon Apr 27, 2020 8:56 pm
Most epoxy primers are sandable if allowed to cure. The time for this varies, anywhere from 24 hours to a week. Up until that time it is usually possible to paint directly over the primer without sanding. It is important that you read the Technical Data Sheet for the product you have, to get the correct details for that product.

Generalising again, most epoxy primers come as the epoxy part and an activator. Some require the addition of a reducer which may be increased if using as a sealer. Again, consult your TDS because products differ, even from a single paint manufacturer.

If you've sanded the paint off your bar then the surface is going to be quite rough, compared to a new bar. Even if you've finished off with say, P400 on a DA, there will likely be deeper scratches. These will show through if you don't use a primer. Some epoxies, when mixed as a sandable primer will be thick enough to fill these scratches but without knowing what product you have I can't say. I'd suggest though, that that is why you've been advised to use a 2K primer - for its ability to fill.

If you use a 2K primer then there is no need for the epoxy. In the same vein, if the epoxy is thick enough then you don't need the 2K. In either case, if you've sanded the plastic then the adhesion promoter is optional - I only use it on very smooth new plastics or on plastics that have a pattern that might be damaged if sanded.

So, if it were me, how would I address this job? Sand off all existing paint with not coarser than P180 on the DA then take out most of those scratches with P320. Clean and then prime with a 2K primer. Sand back to P800 wet or P500 dry on DA and then apply base and clear coats.
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PostPosted: Tue Apr 28, 2020 1:29 pm
If you sand to the plastic, you need adhesion promoter before any primer.
I tried epoxy once over sanded plastic and it all peeled two yrs later.
I'll never do that again.
JC

(It's not custom painting-it's custom sanding)

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PostPosted: Tue Apr 28, 2020 3:33 pm
That's a good point JC. Some epoxies are suitable for plastic, some are not.

The one I use is, but I rarely use it on plastics. On new, OEM quality bars I scuff with red Scotchbrite and then 2K or 1K prime as a sealer. Certified aftermarket usually comes pre-primed or treated so just wash and water/alcohol clean those before a light scuff and prime/seal. For other aftermarket I heat and wash then water/alcohol clean before using a special plastics primer. For repairs I sand and 1K or 2K prime, 2 coats.

I only use adhesion promoter where the surface of the plastic needs to be preserved and that isn't often. I've just finished a 4 litre tin of AP that I bought some 10 years ago. We do 4-5 bars/week, mostly repair, and the only time I've ever had an adhesion problem was on a body kit that came off Ebay. Absolute rubbish plastic but eventually solved with the special plastics primer that Dupont developed specifically for that situation. With regular customers i do get to see bars that we've done previously, as they come back with new damage, and there are no adhesion issues, ever.
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PostPosted: Wed Apr 29, 2020 6:55 am
Do I use adhesion promoter before or after body filler. I got a can of bulldog which is all the supply house had.

Also another thing I'm worried about is the tech sheet on the primer I got which is a epoxy primer sealer, It says to wash the surface with soap water , clean it with wax and grease remover,scuff it down
And clean it again with wax and grease remover.

Bulldogs specifically says after applying it not to use wax and grease remover. But I'm going to be priming over the bulldog and my primer says the opposite what do you think.

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PostPosted: Wed Apr 29, 2020 11:45 am
heyhi1 wrote:Do I use adhesion promoter before or after body filler


Neither. Read my earlier posts. AP is for bare plastic only.

Work through the cleaning procedure before you use the AP, if you must. After the AP, which should be no more than a single coat, wait 30 minutes then proceed directly to the following coat of paint.
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PostPosted: Thu Apr 30, 2020 2:18 am
NFT5 wrote:
So, if it were me, how would I address this job? Sand off all existing paint with not coarser than P180 on the DA then take out most of those scratches with P320. Clean and then prime with a 2K primer. Sand back to P800 wet or P500 dry on DA and then apply base and clear coats.


Here is where my confusion is coming in. I haven't started yet and this is the first time I'm doing it so I'm running a lot of situations through my head to prepare myself.
The paint is actually peeling in a lot of different areas
So in my head I'm running through the situation that I might have to sand it down to bare plastic to avoid this again. How would I know if I should sand down to the previous primer and fill sand and prime on top of that or saying the past the previous primer and start completely from the beginning

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PostPosted: Thu Apr 30, 2020 3:48 am
Ok, the first thing is that the thicker the coatings (filler, primer, paint, whatever) over the plastic are then the greater the likelihood that it will chip badly or fail. So, you want to keep coating thickness to, within reason, a minimum, obviously bearing in mind that a certain thickness of each layer is necessary for it to perform the way it was designed.

Generally, factory paint sits in the range of 70µm to 130µm or roughly 3-5 thousands of an inch. Resprays are usually thicker because of the reliance by many painters on primer to take out variations in the repair surface and/or excessive coats of clear. Typically, though, film build on a professionally done respray will sit around 180-200µm. Added to the original, factory film build of 100µm (average) this will take the total to close to 300µm. Most paint companies specify that this is the upper limit. So, if it's already had two paint jobs then adding a third is more likely to lead to failure and no paint manufacturer will warrant paint this thick.

Your bar has had the two paint jobs and is probably close to the limit. Removing the last job will give you the room you need to keep thickness below 300µm but it is extremely difficult to take off, evenly, layers which are only the thickness of a human hair. The unevenness will show through in your finished work and exposing the different layers when feathering significantly increases the possibility of frying up and edge mapping. Sanding back to the plastic removes those risks and allows you to ensure that each coat is what it should be, which will improve your chances of achieving a job that looks good and will last. So, for maybe 30 minutes work on an average bar you're reducing that risk, and cost, of having to rectify things that can go wrong or, at worst, having to do the whole thing again.

While I might be able to prep and paint 3 bars in a day the average amateur is more l;ikely to spend a whole day on a single bar. That's not a poor reflection on the amateur guy, just that I've been doing it for a long time, every day. So, if you're going to spend a day to fix this bar, why not spend another half hour to improve your chances of getting it right?

Since there is no other damage, other than peeling paint, sanding back to the plastic means that you're going to start the job of repainting with a good substrate. Spending a little time sanding it back nice and smooth and level will make your painting job much easier. It also means that you don't have to put extra layers of various products on over a doubtful surface to try to get something sound. Remember, keeping film build down means that the paint will perform and last much better.
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PostPosted: Thu Apr 30, 2020 6:07 am
This leads me to my next concern which is probably the last one I have.
Let's say I sanded down to plastic, now since it's plastic I have to use adhesion promoter.
I spray the adhesion promoter before body filler so the filler sticks.
After sanding the body filler in any scratches or anything I have to fill I'd have to prime it.

Okay here is where I'm getting nervous, I already have bulldog adhesion promoter, the direction say clean the surface with waxing grease remover and scuff it before applying adhesion promoter. Then it's specifically says do not use wax and grease remover after

Now the primer which is an epoxy sealer primer and sandable specifically says clean surface with wax and grease remover, sand to I think it was 320 , and clean the surface with wax and grease remover again before priming. This is my whole concern The bulldog adhesion promoter says don't clean with wax and grease from over after the promoter is put on. I don't know what to do in between these stages to clean everything and not have any problems with any of the stuff I laid and cause any peeling

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PostPosted: Thu Apr 30, 2020 7:10 am
Why do you need filler?

You sand the paint off and plastic level with P180 on a DA and then go to P320. Use an interface pad so you reduce the chances of digging in. Keep pad level as much as possible. This gives more than enough key to enable mechanical adhesion of a 2K primer. Adhesion Promoter is not required in this situation.

If you did have some imperfection that is too big or deep to sand out then apply filler in that area to the P180 sanded surface. Level and feather with P180 again and finish with P320 before primer. Do not use AP under filler.

If you get "hairy" bits of plastic coming through your primer, stop and allow primer to dry/cure and sand back smooth with P180 before adding a further two light coats.
Chris
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