Minimum hardener to bondo ratio

[bradleyheathhays]

I've using some regular bondo to finish something I'm working on and I'd like to mix it so that it cures as slowly as possible.  Relative to the recommended ratios what's the smallest amount of hardener you can add and still have it cure?

[Tom King]

The only way to make it cure slower, and still maintain enough strength is to lower the temperature of everything involved.  If you really need something to cure slower, use a slow cure epoxy and add filler.  It doesn't sand much differently, and is much stronger.

I had a boat repair shop back in the 1980's with an oversized AC system.  I would cool it down below 65 to do a layup, and then turn it up to normal to cure.  A tornado wiped out that shop.

Bondo is polyester resin with filler.  There is not much slowing it down.  Just not worth the trouble.  

There are some other body work fillers that are good for other variations, like what they call glazing putty that goes on smoother and is for touching up pinholes in body filler.  There are also some much better body fillers than Bondo, but they get more expensive quickly.

[Coop]

My first inclination was that too little hardener and it didn’t cure at all and too much, it would never leave the mixing bowl as a liquid. Glad you cleared that up @Tom King! 

[bradleyheathhays]

I'll show ya what I'm working with here.  I've got about a 15 in tall 3D PLA plastic printed form I'm filling and sanding to smooth.  The 3D printer's layers are .3 mm so those are the lines seen in the pictures.  This picture is after 3 coats of high build filler / primer and 1 sanding with 120.  I've got another 3 coats on now and will sand tomorrow to see how well 6 coats fill the lines in.  I'm sure something else would work better and be less expensive to use, and the putty you mentioned might be the thing.  I've seen putty called spot putty and glazing putty.  Both come in a tube a little wider and shorter than a tube of toothpaste, and one has a hardener and the other doesn't?  I'm not entirely sure which is which.  This finished form is going to be used to make a silicone mold negative so whatever filler I use doesn't have to be high grade...just so it sticks to PLA plastic, which this high build primer seems to do well.  The second picture has some distortion in the form of added dots.  I was trying to get a better view of the small lines between print layers.  Not sure why the spray filler has a tendency to fill the edge of the lines last.  I'm sanding with perfectly flat sandpaper adhered to 1/4" acrylic sheet cut specifically for each facet size.

 

[wtnhighlander]

I think the putty Tom mentioned will work for those tiny grooves. The type that needs no hardener is what I have used for painted projects in the past. Try the putty, then add a final coat of that primer.

[Tom King]

Bondo will do that, but a better quality body filler will save a lot of time and work.  It's a lot creamier, and goes on smoother for thin coats.

https://www.amazon.com/3M-01131-Platinum-Plus-Filler/dp/B005RNEYAM/ref=sr_1_1_sspa?crid=3S7K81FIZ3KSR&keywords=3m%2Bplatinum%2Bbody%2Bfiller&qid=1691493924&sprefix=3m%2Bbody%2Bfiller%2Caps%2C93&sr=8-1-spons&ufe=app_do%3Aamzn1.fos.006c50ae-5d4c-4777-9bc0-4513d670b6bc&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1

Try sanding with finer sandpaper too, like 150 or even 220.

[Tom King]

I like metal spreaders a Lot better than plastic ones too-especially over sharp corners.

https://www.amazon.com/Custom-Shop-4-Piece-Spreaders-Scraper/dp/B07L1BP91V/ref=sr_1_1_sspa?crid=1WGAWPD4BS4PO&keywords=metal%2Bbody%2Bfiller%2Bspreader&qid=1691494411&sprefix=metal%2Bbody%2Bfiller%2Bspreader%2Caps%2C107&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1

[Tom King]

Mixing boards with disposable sheets keep it a lot cleaner with less pills to roll up like sometimes come off cardboard, and are well worth it.  

https://www.amazon.com/Custom-Shop-Multi-Layered-Disposable-Filler/dp/B07L1CTR4R/ref=sr_1_1_sspa?crid=1RB5G9Z0UOB6Q&keywords=body+filler+mixing+board+sheets&qid=1691494522&sprefix=body+filler+mixing+board%2Caps%2C103&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1

[Tom King]

This is a very ambitious project for a first body work job.  If it's to be sprayed with a high gloss finish, every step is important, as is the atmosphere it's to be sprayed in, and the equipment used.  When it comes time to buff it out, you're going to have to stay off those sharp corners or they will burn through causing you to have to start over.

You should be good at it by the time you finish.

[bradleyheathhays]

Thanks for all the links there Tom.  This piece will only be sanded up to 600 or so and then captured in a silicone mold.  Then the mold is used to create the final piece.  So thankfully I won't have to prep it all the way up to being sprayed.

[Tom King]

I use that silicone mold stuff for casting window sills on old houses out of concrete.  I age a wooden model, and pull the mold off of that.  They had a lot better wood 200 years ago than we do today and theirs didn't last, so it makes no sense to go back with wood.

[wtnhighlander]

12 hours ago, Tom King said:

I use that silicone mold stuff for casting window sills on old houses out of concrete. 

That sounds like an interesting process. If you have to do it again sometime, please share!

[Tom King]

I use Yellow Pine framing lumber which is close enough to the old Long Leaf Heart Pine sills they replace.  Make the sill the size I need, burn the surface with a propane torch some, which mostly erodes the soft grain, and then go over it with a Surface Conditioning Tool with a soft wire brush wheel on it.  Suspend that in an outer form that allows enough room to pour the silicone in.

I have some rollers that do distressed wood for the top, but so far everyone has agreed with me to do the sloping tops smooth so they can shed water easier, and most people won't look at that part anyway.

Once the silicone mold stuff cures, the outer form is pulled on.  The silicone is stretchy enough to pull off the form, which was coated with a Lecithin release agent that comes in a spray can.  

A metal pan is folded up to hold the concrete from running down in the wall and put in place.  I usually use trim coil vinyl coated aluminum because they usually have some tight folds.  The form used to make the silicone mold is put in place on the outside, and the concrete poured in place.  The form is taken off a few days later.

That is left to cure for six months or so, then coated with epoxy, and painted.  It ends up looking just like an old wooden one.

It's not as bad as it sounds like.  I don't want to bother to do stuff that only lasts a few decades.  Paint today is so good it's like putting a plastic sack over a wooden window sill.  Water is going to get in, and stay in, which shortens the life of any wood in that postion.

I made some out of Live Oak back in the early '80's thinking they would last a long time, but they didn't.  Treated wood doesn't stay treated long enough either under paint.

I have some pictures somewhere, but I don't think of the whole process. When I'm working, I don't often think to slow up enough to take pictures.  Almost all the pictures on my old website were taken by someone else.

[Tom King]

Found a couple:

I don't know why that one picture is upside down, and don't know how to change it here.

The wedge on top is to keep the sash open long enough for the concrete to cure.  It was probably just scrap laying around.

[Tom King]

Here is the first one I did, probably in 2016.  This one was so close to the brick on the ground below that I thought it was worth a try.  That one worked out so well that I never did any more any other way after that.  This was before I made them match the old ones, but these had been replaced so many times that we didn't know what the originals looked like any way.

This is an original 1850 window unit.