Bondo Work And Primer Application 101

[fhernand]

This thread is the third 101 help thread covering bondo application and primer. There are many techniques on spreading and finishing bondo. The technique herein, is quite easy. Of course, it also takes skill and much practice to get your bondo work straight with no waves. The one thing I would emphasis for straight bondo work is to use you’re senses…feel and sight. As you sand notice the high and low spots. Any high spots found, take down with a pointed hammer and dolly. If there are low spots, raise with hammer and dolly. The safety tips:

1. Use a well ventilated area when using bondo.
2. Protect your hands from thinner and other chemicals.
3. Wear that dusk mask when sanding bondo.
4. The rest is common sense.

The tools need are shown on the table below:

1. One 9 inch and 16 inch sanding blocks.
2. 36, 40, 80 grit straight-line sanding paper (I recommend 3M paper since it last much longer).
3. Pointed end hammer and dolly.
4. Bondo mixing tray. (I use a door window off a 1964 356 coupe…I know kill me later guys) It’s easy to clean up the left over bondo.
5. Bondo and hardener. I recommend Evercoat Premium Lightweight Filler. Great stuff very easy to sand and spreads very smooth.
6. Plastik Honey. I use this stuff mixed in with my bondo. The stuff work great for avoiding small air bubble pin holes in your bondo after it has dried. Save time and the end.
7. Plastic Spreader. For large areas I recommend a larger width spreader.
8. Metal container and metal putty knife to dish out and mix the bondo. I keep a little thinner in the metal container to do my bondo clean up.
9. One can of black guide coat. This stuff keeps your primer and bondo work honest. I use this to find my low spots. Any low spots left, the sand paper will not scratch the guide coat thus, leaving a visible low spot with guide coat inside. Notice the PPG Urethane Primer Surfacer NCP 280 below.



After having sprayed and letting dry the two coats of Epoxy Primer, I went ahead and used a marker or pencil to mark ALL my high and low spots. I used a pointed hammer to work down the high spots on the hood. The low spots I worked up using the same hammer and dolly technique. I found low spots all over the front end of the hood. This happens when Porsche hoods are not properly closed with hands causing dents. Always close hoods using both flat palms on each front corner of the hood. Because I found so may low spots, I decided to spread a nice even coat over the entire front of the hood. I mix the Plastik Honey, Bondo and Hardener all at once. Make sure that the bondo is WELL mixed until the ENTIRE spread is the same color of the hardener. Mix very well. If you leave area not well mixed, this will cause serious problems down the road.




Spread your bondo evenly and with gentle pressure using a larger width bondo spreader. Remember you only have so much time to spread the bondo before it starts drying. The hotter the outside temp. the faster the drying time. Concentrate on spreading the bondo evenly.



I use 36 grit to cut down my first spread of bondo. Use a the large 16 inch sanding block. The larger block will help you get a flat hood. Keep it flat and let it do the work for you. As you sand, notice the low and high spots. If needed use the pin hammer to knock down the high spots. It take much more time using a hand sanding block than an air sanding tool. Remember you do not want to add weight to the hood while you are sanding. You will not get a flat hood using too much pressure while sanding. The metal in this area of the hood is very flimsy. I recommend a hand block with little pressure. Trust me on this one.


After sanding using a block and keeping it flat, the block will tell you where the low spots are. Go back and mix some more bondo and fill those low spots. After the second coat, I use 40 grit lightly to take down the large 36 grit scratching marks.


After I have finished sanding the second coat with 40 grit, I lay down a third glaze coat of bondo. This time and double the amount of Plastik Honey to fill-in any remaining pin holes. Sand down with 80 grit and you are ready for primer.


PPG NCP 280 Urethane Primer Surfacer is a high quality primer. It lays down very smooth and go on very thick. You will need to use a large tip on your spray gun to apply this stuff. It sand great and is very durable. Follow the recommended mixing ratios closely. The stuff stinks but very good stuff. The idea here is to use a good primer surfacer. I applied three WET coats of primer. After the primer has dried for several minutes, I use a guide coat over the entire hood. The next day the hood is ready to be blocked using 220 grit. Use the large sanding block on all your flat spots. Be sure to keep your block flat at all times during the sanding. After blocking the entire hood sanding out any minor low spots, I spray another two coats of primer and guide coat a second time. Once this has dried, you are reading for final sanding using 400 grit.



I will post another thread in the future on final sanding, paint preparation and paint. Hope you like the thread.

Regards, Freddie

[sp441]

What, if anything, needs to be done different when working with fiberglass? Keep up the good work.

[fhernand]

SP441, if I was working with fiberglass and the glass and paint was cracked, I would use a fiberglass paint remover. Otherwise, I would simply sand the finish coat to repaint. But the bondo work is the same. On fiberglass I recommend using a Polyester base primer. This stuff works great on fiberglass. I would like to see you project?

[Zeke]

Freddie, it looks like you sanded thru your epoxy primer in places. I know this happens to me and I wonder if this causes problems down the road as the epoxy is a good moisture proof coating and the filler/primer is not. You did mention in another thread about using a thinned epoxy as a final sealer. Is this how you manage the spots sanded thru?

BTW, your "101" series is most excellent, even for us experienced guys. Thanks again.

[Nitrometano]

Wao, that's interesting. I'm aplying Bondo right know and I never heard about the Plastick Honey and their function. Also I never minded to use a glass surface to mix the Bondo and the easy cleaning. From now on I will try to use Fernand technics.

[fhernand]

Zeke, on the bare metal spots I cover when I spray the Urathane base primer. That is why I like this stuff. It is recommended for bare metal use also. However, if the metal spots showing through were any larger, I would spray two coats of the epoxy.

[fhernand]

SoCal, place the dolly on the outside and hammer gently with a flat hammer on the opposite side. Make sure your dolly is flat and the hammer is flat this is very important. If it's a small ding you can get away with not stretching the metal. If you do stretch the metal than we have to apply a different tech. using heat to shrinking the metal back. Best wishes.

[sp441]

fhernand, I am hoping to update my project posting on here soon. I ran into a snag with my sandblaster so I am waiting for parts. The fiberglass parts I have are all new, hood, fenders and both bumpers. Hoping to have it primed and painted by April. A nice quiet speed yellow.

[fhernand]

SP411, can't wait to see the finish product. That is a nice color.

[86 911]

Great Thread. Tell me more!

[rw7810]

Freddie - I'm ready for more. You did such a great job on the 1st three articles.

I will post another thread in the future on final sanding, paint preparation and paint. Hope you like the thread. Regards, Freddie

[kycarguy 935]

Very nice write up.

[adomakin]

Freddie, is it cool to just go straight to primer from 80 grit?

[fhernand]

Hi guys, it's been awhile for me since I have been on the forum. I'am looking foward to completing the series of articles. I will be writing a few more on Painting Prep, Painting and Buffing. I had to make a sudden stop on my 67 s project due to new job and marriage! I have used my own car to illustrate these write ups. Fortunately, I'm slowly getting back on my car now and will be moving foward on some more write ups soon.

Adomakin - I highly recommend finishing your bondo work with 150 grit or 220 before bondo. If you prime over 80 grit, you may get some "swelling" after you paint and you will see all those rough 80 grit groves.

[Zeke]

Originally posted by rw7810
Freddie - I'm ready for more. You did such a great job on the 1st three articles.

Absolutely. And I'm afraid that I have mistakenly given credit to these fine articles more than once to a wrong name. I will bookmark this time.

In answer to the 80 grit, a lot of people like to sand the filler out pretty smooth as Freddie says. Then, the use of a catalyized polyester primer will help protect against "sand scratch swelling" as the primer is made from the same resin as the fillers are. They become one. But, don't forget to use sealer on poly primer. I found out the hard way that poly primer is very porous even though is looks/feels smooth. It just sucks up and dulls your paint making for more work in the color sanding and polishing phase. The old lacquer ffiller primers were the worst for telegraphing the surface beneath, especially in time.

Would you agree, Freddie?

[fhernand]

Milt, I agree. Use a high grade sealer over your primer and before painting. I good primer like the one I described on the write up will prevent any "swelling". Most shops out there will primer over 80 grit then use a poly filler and will cover the 80 grit groves. If you do so, then you will have to apply several coats of poly filler and sand between coats. I like to use finer grit to sand my bondo this way I will not only elimate any chances of swelling and the use less material but also, make sure my bondo is straight. You can always feel your work much better when it's sanded with finer paper. Regards.

[Nitrometano]

this is a great article I'm learning a lot. I used some of the information here in my RX-7.

[adomakin]

cheers Freddie/Milt. I have always gone finer than 80 before priming, it was just freddies wording in his write up that made me query it. Im amazed that lots of shops go from 80 to primer then poly filler. can you really bury 80 scratches with primer and poly filler?

[RX911]

adomakin;

How do you sand a curved surface? I like the long sander but how is it used to get a flat surface on a curved body panel?

tom

[adomakin]

tom, you wanna be asking freddie really but heres my take. your long sander is redundant on anything other than flat or very lightly curved panels. for curved/shaped panels and depending on the type of curve i would either use an adjusatble sanding block or a small hand block and just work the main shape out with 40 grit, then 80 and so on. somethimes ill just use paper on its own.