In another thread I had some questions about painting stripes, in my case to help distract from slightly mismatched paint on my hood.

That prompted a thought....in originally spraying my hood I didn't get things right the first time, or even the second and had to respray. In my case here's the sequence of events:

1. Strip hood to bare metal
2. Two medium wet coats of epoxy for corrosion protection
3. Two coats of high build primer, then block sand with 320 grit
4. Two more coats of high build primer, then block sand with 320 followed by 400
5. A coat of 50% thinned epoxy primer, for use as a sealer coat.
6. Two medium wet coats of SS metallic paint. Notice tiger stripes when done, sand with 400 grit and grumble while doing it.
7. Two more medium wet coats of metallic paint, again see tiger stripes, kick trash can, sand with 400 grit.
8. Use paint from new can (that turns out to be slight color mismatch with other panels), this time avoiding tiger stripes.
9. Follow with three medium wet coats of clear.

That's where things stand now. I'm not a pro, but it sure seems like a lot of layers on there. If I scuff the hood and lay down a coat of stripes, then another three coats of clear, am I risking too much paint build up? What's the rule of thumb on how many coats of material are too many?

The number of layers is not the point, although what you've outlined above is way too many. What is important is the total paint thickness.

Too thin and you risk insufficient protection, too thick and the risk is propensity to chip, crazing and cracking. Other issues include residual solvent related problems which can lead to layer separation.

My rule of thumb is that any existing coating thickness greater than 3 existing coats becomes risky.

Consider what the paint companies recommend. From the top down, clearcoat is typically specified with a Dry Film Thickness of around 60μm. Basecoat up to about 30μm and HS primers up to about 250μm. That's 340μm total and these are maximums. Add maybe 40μm for a sealer and you're up to 380μm, or let's say 400μm in round figures.

I do this for a living and I pay for the materials, so my DFTs are generally well below the maximums. On a bare metal start I typically achieve around 200μm total and this really is optimum. Thick enough for good protection, thin enough for flexibility but still within the specified thicknesses and therefore qualifying for the paint company supported lifetime warranty that we offer.

Paint thickness gauges are not that expensive any more and use, on each layer, can ensure that you are putting the paint on within its design parameters, meaning that it will last and perform at its best. Not only that but a PTG can save its purchase price in materials costs in a single restoration.

Striping and mottling can be easily avoided with proper gun setup and technique including ensuring correct distance, holding the gun perpendicular to the surface and use of a drop coat. A practice panel would have saved you the cost of one or two layers of your base and avoided much of the problem you now have. There is no need to clear between background colour and stripes, saving you more money and overall thickness. The sealer coat probably wasn't necessary, although I acknowledge it was well thinned. It could have been used instead of your second two coats of high build primer if your prep work was good enough.

TBH, if this is a daily driver and you want it to last for some time I would sand back to primer and do it again. More expense? Yes, but cheaper and less trouble than having to do it all again when it fails.

I'm just curious as to why you think you need to use all those layers and products, on a car hood? I counted 14 coats which could be >20 mils

Last I recall, PPG won't warranty over 12 mils (305 µm), and recommends stripping before any further coats.

Every coat gives new opportunities for problems. Might be fine for doing a museum job but why take this risk for a car you drive? Also, do we really think a car hood is going to rust if you don't epoxy it?

My process:

1. Bare metal
2. HP-5310 primer, build then block down
3. Color
4. 2-3 coats of clear

Maybe 7 mils when done.

Easy process, show quality results, and a long lasting chip resistant finish. What more do you need?

1. Strip hood to bare metal
2. Two medium wet coats of epoxy for corrosion protection
3. Two coats of high build primer, then block sand with 320 grit
4. Two more coats of high build primer, then block sand with 320 followed by 400
5. A coat of 50% thinned epoxy primer, for use as a sealer coat.
6. Two medium wet coats of SS metallic paint. Notice tiger stripes when done, sand with 400 grit and grumble while doing it.
7. Two more medium wet coats of metallic paint, again see tiger stripes, kick trash can, sand with 400 grit.
8. Use paint from new can (that turns out to be slight color mismatch with other panels), this time avoiding tiger stripes.
9. Follow with three medium wet coats of clear.

Step 3.
Try shooting 3 coats on build primer.
At this stage you are trying to get the panel straight and you should be using a much coarser grit paper when block sanding. This will remove nearly all of the build primer just leaving it in the low spots and thus prevent too much build (paint thickness) in your overall process. Generally using an epoxy in a contrasting color to your build primer will help you "see" when you are getting close.

Tiger stripes were addressed in another post I believe.

All this points to the need of having the proper equipment (adequate CFM air supply, decent spray gun with proper tip set, good lighting and ventilation, etc.), knowledge to set the gun up properly and a consistent spraying technique.

Practice on an old car hood, fender, etc. until you are sure the paint is going on the way it should. Holding the gun consistently the same distance from the panel while keeping it perpendicular to the surface (not tilting forward or backward) and maintaining the same percentage of overlap on the entire surface.

It gets real expensive and labor intensive trying to learn while spraying your actual project!

NFT5 wrote:The number of layers is not the point, although what you've outlined above is way too many. What is important is the total paint thickness.

Too thin and you risk insufficient protection, too thick and the risk is propensity to chip, crazing and cracking. Other issues include residual solvent related problems which can lead to layer separation.

My rule of thumb is that any existing coating thickness greater than 3 existing coats becomes risky.

Consider what the paint companies recommend. From the top down, clearcoat is typically specified with a Dry Film Thickness of around 60μm. Basecoat up to about 30μm and HS primers up to about 250μm. That's 340μm total and these are maximums. Add maybe 40μm for a sealer and you're up to 380μm, or let's say 400μm in round figures.

I do this for a living and I pay for the materials, so my DFTs are generally well below the maximums. On a bare metal start I typically achieve around 200μm total and this really is optimum. Thick enough for good protection, thin enough for flexibility but still within the specified thicknesses and therefore qualifying for the paint company supported lifetime warranty that we offer.

Paint thickness gauges are not that expensive any more and use, on each layer, can ensure that you are putting the paint on within its design parameters, meaning that it will last and perform at its best. Not only that but a PTG can save its purchase price in materials costs in a single restoration.

Striping and mottling can be easily avoided with proper gun setup and technique including ensuring correct distance, holding the gun perpendicular to the surface and use of a drop coat. A practice panel would have saved you the cost of one or two layers of your base and avoided much of the problem you now have. There is no need to clear between background colour and stripes, saving you more money and overall thickness. The sealer coat probably wasn't necessary, although I acknowledge it was well thinned. It could have been used instead of your second two coats of high build primer if your prep work was good enough.

TBH, if this is a daily driver and you want it to last for some time I would sand back to primer and do it again. More expense? Yes, but cheaper and less trouble than having to do it all again when it fails.

interesting
Jay D.

'68 Coronet R/T wrote:

1. Strip hood to bare metal
2. Two medium wet coats of epoxy for corrosion protection
3. Two coats of high build primer, then block sand with 320 grit
4. Two more coats of high build primer, then block sand with 320 followed by 400
5. A coat of 50% thinned epoxy primer, for use as a sealer coat.
6. Two medium wet coats of SS metallic paint. Notice tiger stripes when done, sand with 400 grit and grumble while doing it.
7. Two more medium wet coats of metallic paint, again see tiger stripes, kick trash can, sand with 400 grit.
8. Use paint from new can (that turns out to be slight color mismatch with other panels), this time avoiding tiger stripes.
9. Follow with three medium wet coats of clear.

Step 3.
Try shooting 3 coats on build primer.
At this stage you are trying to get the panel straight and you should be using a much coarser grit paper when block sanding. This will remove nearly all of the build primer just leaving it in the low spots and thus prevent too much build (paint thickness) in your overall process. Generally using an epoxy in a contrasting color to your build primer will help you "see" when you are getting close.

Tiger stripes were addressed in another post I believe.

All this points to the need of having the proper equipment (adequate CFM air supply, decent spray gun with proper tip set, good lighting and ventilation, etc.), knowledge to set the gun up properly and a consistent spraying technique.

Practice on an old car hood, fender, etc. until you are sure the paint is going on the way it should. Holding the gun consistently the same distance from the panel while keeping it perpendicular to the surface (not tilting forward or backward) and maintaining the same percentage of overlap on the entire surface.

It gets real expensive and labor intensive trying to learn while spraying your actual project!

you have received some very good info here in several posts, good luck.
Jay D

chris wrote:I'm just curious as to why you think you need to use all those layers and products, on a car hood? I counted 14 coats which could be >20 mils

Last I recall, PPG won't warranty over 12 mils (305 µm), and recommends stripping before any further coats.

Every coat gives new opportunities for problems. Might be fine for doing a museum job but why take this risk for a car you drive? Also, do we really think a car hood is going to rust if you don't epoxy it?

My process:

1. Bare metal
2. HP-5310 primer, build then block down
3. Color
4. 2-3 coats of clear

Maybe 7 mils when done.

Easy process, show quality results, and a long lasting chip resistant finish. What more do you need?

My understanding is you should always put epoxy over bare metal, which is why I did step two. The block sanding steps certainly could have been done initially with coarser grit than 320, as that took me ages to get done.

I certainly didn't intend to shoot three layers of basecoat, but the stripes told me different. I did shoot a couple of test panels beforehand with no issue, though they were vertically oriented and as I've learned, striping seems to be more of a problem with a horizontal surface-- at least for me. Meanwhile I'll keep my fingers crossed that no cracking takes place.

Epoxy is not a requirement for bare metal. There are zero cars in production today that use epoxy as part of the OEM process on bare body panels. What is required is a DTM primer. HP-5310 is an excellent buildable DTM primer. There are others as well.

Everyone has to make a decision they're comfortable with. I'm happy with my version. I do use epoxy on anything underneath the car, where it is worth doing.

etch primer is also an alterative if you don't have or use dtm primer. i'm a little confused ( normal ) but you have talked about Single Stage and then Base Coat to me they are different, or usually the way we talk here. if I went to my jobber and asked for Single Stage Base coat i'm sure I would get a wide eyed reaction. if its truly a S/S, then as mentioned before a drop coat properly applied should eliminate the problem. its not going to cure the cause of the problem but it can make the job look good. for true base coat if mixed and properly applied and of good quality you should never have the problem. I could mix some silver or gold metallic Chroma Base give my wife the gun and tell her to spray a door with her eyes closed. it would probably come out near perfect. I just don't see how anybody could screw it up its so easy to use.
Jay D.

chris wrote:Epoxy is not a requirement for bare metal. There are zero cars in production today that use epoxy as part of the OEM process on bare body panels. What is required is a DTM primer. HP-5310 is an excellent buildable DTM primer. There are others as well.

Everyone has to make a decision they're comfortable with. I'm happy with my version. I do use epoxy on anything underneath the car, where it is worth doing.

I stock many products, including some you sell, and use many types depending on job application. PPG HSP primer was CAT approved for machinery undercarriages as a stand alone product. Almost zero mention of this product on hobby sites. No restored car will see the abuse of an excavator. The commercial line is less expensive than the automotive line but performance is not inferior. PPG CRE or VP 2050 are great products but rarely get mention on most sites; filler applied over both is no issue and are capable of high build, no shrinkage. Most pros and jobbers do not know about them unless you speak to the right people in the industry. Find a company rep to visit a shop, or call a tech line. Some jobber employees are more than "counter people", they actually have training experience and use the products themselves.