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Technical Body Filler Basics

Discussion in 'The Hokey Ass Message Board' started by K13, Jan 11, 2020.

  1. There always seems to be a lot of questions surrounding body fillers and putties so I thought I would put together a thread to cover some of the basics. I will give some general info about fillers and putties make up, differences, etc. and then a few tips. I apologize if some of the pictures aren't the best as I was making some panels for work and just did the stuff for this quickly while I was working on those. Keep in mind that all these products are designed for the Collision Industry and are formulated to increase production and reduce problems not for ease of use for hobbyists.

    Pretty much all body fillers and putties in North America are what are known as lightweight products. They consist primarily of 3 components: Polyester resin, Talc and Microspheres. Different companies then add different components to increase performance aspects but all contain these three basic components.
    filler 4 (2).jpg

    Resins: The vast majority of fillers and putties use polyester resins. There are a few exceptions where hybrid resins are used but I won't get into that here. Generally more expensive fillers will use more expensive resins that will flow better and sand better. Putties use a unique resin that cures to a tighter and denser film which provides a more stable and more solvent resistant surface which is why they were developed to provide a better surface for primer application than what most fillers can provide.

    Talc: Talc is the main body of a filler. Think baby powder as a common talc product. The are different sizes and shapes of talc used in various products. There are two basic shapes of talc used in body filler and putty: Spherical (top) and Platelet (Bottom).

    Round talc is what is used in body fillers. It is easier to sand due to the fact that you are generally sanding a smaller area of the mineral.



    Platelet talc is usually used in putties because it is easier to spread and gives a smoother finish and a denser surface structure. The easiest way to think of the differences is think of trying to cover the bottom of a box with balls or sheets of paper. You will get more spaces between the balls than the sheets of paper and thus not as dense a surface. Combine this with the special resin used in putties and you get a surface that does not absorb nearly as many solvents as body filler during primer application.

    Microspheres: Mircospheres are tiny hollow glass or plastic bubbles that are added to fillers and putties. These are what makes a filler a lightweight product. Micropsheres main purpose is to increase sandability. Cutting through the hollow sphere is much easier than the solid talc particles. But there is obviously a trade off in they are not a solid particle that will add stability and density to a surface like talc will. Filler companies use this to their advantage to make cheaper fillers that sand really well without having to use more expensive resins and finer talc grinds by adding more microshperes which sand better but don't provide as dense or hard a surface.
  2. Ok now we will talk a bit about mixing and applying. With the exception on one product all body fillers are a 50:1 mix ration or 2% weight by volume. You should always kneed the hardener tube and stir your fillers before using to ensure everything is mixed up as they will settle and separate over time. The easiest way to eyeball the correct amount of hardener to use is with a simple ribbon method. Place your filler on your mixing board and let it self level. You can bang on the bottom of the board to speed this up. Then depending on the size of the puddle you run a ribbon of hardener across the puddle. A 2" puddle you run the hardener half way across, a 4" all the way across, 6" all the way across and 8" all the way across and then another strip half way across. This method has been tested with a scale and is well within the accepted ranges of hardener use.
    Filler 5 (2).jpg

    Note: Do not use porous surfaces like cardboard to mix fillers or putties on as they will absorb the products and can change the ratios.

    Over and under catalyzation are the cause of the vast majority of issues with filler. Too much can lead to filler becoming brittle and cracking. It will also increase the chances of pinholes forming. Under catalyzation can led to shrinking and movement later on down the road which can lead to mapping of the repair area. so make sure you try and keep pretty close to the about ratios.

    The best way to mix the product is to try and fold it over itself rather than stirring. This tends to add less air to the mix. You want to mix it until you get a uniform looking product. You do not want streaks of hardener in the mixed product. It should look like this

    Always use the hardener that is provided by the manufacturer. About 3 years ago there was a world wide BPO shortage which is the active ingredient in hardener and some companies adjusted their formulas to compensate so using a different brand hardener could result in over or under catalyzation even if the above method or a scale is used. Also other than one Evercoat product that uses different speed hardeners there is no difference in different colour hardener. They are made different colours so they can be easily seen in different coloured fillers.

    Now you are ready to apply the filler. The surface needs to clean and sanded. 80 grit is now pretty much the standard recommendation for all filler companies as body shops are trying to eliminate coarse grit paper so this is what fillers are being designed to go over. There is some belief that coarser grits mean better adhesion but that is no longer the case and some of the newer filler will actually perform worse over heavier grit scratches so 80 grit is the best bet. Blow the surface clean. If you want to wipe it down with a solvent make sure to use a fast evaporating solvent like acetone so there is no residue left on the surface.

    A little tip when you have finished mixing your filler is to spread it out over your mixing board rather than leave it in a pile like the picture above. This will do two things. It will slow the catalyzation so you will have more working time and it will let any air you have added during the mixing process escape.
    Filler 2.jpg

    When applying your filler to the surface you want to make sure that the first pass is pressed right into the metal. We call this a wet coat. The idea behind this is it ensures you get the filler pushed down into the sand scratches. This will obviously help with adhesion but it will also reduce pinholes as air doesn't get trapped between the filler and the bottom of the sand scratch grooves. Don't just glop the filler over the area. Once you have pressed it into the surface you can build on top of that. This isn't a great picture but your first pass should look like the one on the left not the one on the right. You don't have to let it cure like that before adding more but you don't want your first pass to look like the right.
    Filler 7.jpg
    A little closer look.
    filler 8.jpg
    Let it dry and sand, reapply as needed. Once you have filler down you should not use solvents over top of it as they can be absorbed and then kicked out later causing failures. Just blow it off and reapply.

    Fillers should not be applied any thicker than 1/4" and putties 1/8".

    I missed adding this the first time but I wanted to make sure it was here. It is NOT ok to use regular fiberglass resin to thin fillers and putties. The resins are not the same and required different types of hardeners to catalyze. Using fiberglass resin will almost always result in shrinking as the resin does not get fully cured by the BPO style hardeners. The only ways fillers and putties should be thinned is with specific resins designed for thinning (Plastic Honey, Super Charger) or mixing a thinner product with a thick like using a putty to thin filler.

    EDIT: I should have added this yesterday these are only guidelines you should ALWAYS read the technical data sheet of any products you use for the best information on what is recommended by a manufacturer.

    That's the basics and I hope it helps. There are lots of other aspects to this so if you have any questions please let me know.
    Last edited: Jan 13, 2020
  3. Roothawg
    Joined: Mar 14, 2001
    Posts: 19,922


    Cool. You work for a filler mfg?
  4. Thanks, Yes I do.
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  5. 40FORDPU
    Joined: Mar 15, 2009
    Posts: 2,843

    from Yelm, Wa
    1. Northwest HAMBers

    Can't add to your in depth coverage of the use and composition of filler..but on this topic, the best use of filler is to use the least amount possible.
    A must have is a hammer and dolly asst. getting out the majority of damage prior to filler.
  6. Chicster
    Joined: Aug 5, 2018
    Posts: 152

    1. Missouri H.A.M.B.ers

    Good info.
    arkiehotrods and K13 like this.
  7. blowby
    Joined: Dec 27, 2012
    Posts: 6,744

    from Nicasio Ca

    I just did my whole heap without using any putty. When do you use putty?
  8. flatford39
    Joined: Dec 3, 2006
    Posts: 2,625


    Not to hijack this thread but the pick up in your avatar is the tits!!!
    K13 likes this.
  9. For the most part putties are a collision industry product unless you are looking for a thinner consistency of product as we tend to be doing completes and using a high build primer over the entire car. Putties were designed to go over fillers to reduce the amount of primer needed and to allow paint edge to paint edge application which traditional fillers are not designed to do. Even collision shops are starting to move away from it as the newer fillers are creating the dense surface structure that putties were known for and can go paint edge to paint edge.
    arkiehotrods, AndersF and blowby like this.
  10. bill gruendeman
    Joined: Jun 18, 2019
    Posts: 333

    bill gruendeman

    Thanks for the information and setting me straight. I have questions on using filler in lower temperatures ( Minnesota in the winter it hard to keep the shop hot ) can you give a little tutorial on that, thanks
    K13 likes this.
  11. squirrel
    Joined: Sep 23, 2004
    Posts: 46,448


    Wait till spring, or move to AZ, or get a better work space with heat and insulation?
    dan griffin, Mark Yac, RICH B and 4 others like this.
  12. Chgo Sox Fan
    Joined: Apr 16, 2019
    Posts: 11

    Chgo Sox Fan

    Great info. Thanks
    K13 likes this.
  13. nor6304
    Joined: Aug 23, 2009
    Posts: 217

    from Indiana

    Yes Thank you for info
    K13 likes this.
  14. Roothawg
    Joined: Mar 14, 2001
    Posts: 19,922


    You are correct. This was taught for years. I still thought this way until this post. I appreciate the effort to school the laymen. I am self taught, so any info is huge.
  15. WB69
    Joined: Dec 7, 2008
    Posts: 1,248


    Thanks for the info. Makes for more confidence when adding the hardener. At least for me.
    osage orange, K13 and hotrodjack33 like this.
  16. squirrel
    Joined: Sep 23, 2004
    Posts: 46,448


    The hardening time is related to the mixing ratio, as well as temperature. When the ratio is right, and the temperature is in the suggested range, what is the hardening time?

    Can the ratio be adjusted a bit to compensate for temperature? If so, how much?
    osage orange likes this.
  17. These both touch on a similar subject so I will try and answer both.

    Body fillers are a exothermic reaction so yes the outside temperature does affect their cure times. Body fillers are designed to be used in temperatures between 60F/15C up to about 100F/38C so extremes either way can cause performance issues. The temperature we test all our products at is 72F/22C and at that temperature you should have between 3-5 minutes of working time and about 20 minutes dry to sand time. Putties cure faster than fillers to help achieve that denser surface structure so keep that in mind when using those. You can play with the hardener amounts a little but it is not much about 10% either direction but that is 10% of the hardener amount not up to 10% of the total mix. It's a pretty small window.

    For colder climates (it's -18F here today and falling so I can relate) if you are not able to get the application surface and ambient air temperature up to 60F for at least the application and cure time you should really be waiting until you can. You can use heat but you have to be very careful. Two things can happen when applying heat. One is if you use too much you can cook the styrene out of the filler which results in an unstable finished product. The second is you can cure the top layer of the filler way faster than the inside and can trap solvents and air which can also lead to failures. The only external heat source we recommend is IR (not a common hobbyist item) because it heats from the inside out and tends to avoid these issues but even with that you need to be very careful to not overheat. The surface of the filler should never exceed 125F/52F. I know there will always be some guys that say they have been doing it in temperature below this for 40 years and never had an issue but they are rolling the dice. For a hobbyist it's not worth the risk of waiting for a few months.

    For hot climates there is not a lot you can do with traditional products to slow them down. As I mentioned above spreading the material out on your board will slow catalyzation times and give you a little more working time, you can reduce the amount of hardener used a little bit and you can just apply smaller amounts of product at a time. There are some newer products on the market that do allow the use of slower speed hardeners (totally different mix ratios) as well as slow cure fillers that use the traditional hardeners and mix ratios that can be used alone or added to certain fillers to slow the cure time. I don't want to turn this into a sales pitch for specific products as I just wanted to provide some basic information for people but if any of you are interested I can provide some more detailed info.
    Last edited: Jan 12, 2020
  18. Fortunateson
    Joined: Apr 30, 2012
    Posts: 3,240


    This is a great thread. First we had paintguru and now we have fillerguru! I think we all appreciate the pros explaining things in a way that the hobbyist can understand and enable us all to do better quality work.
    Could you explain products such as Allmetal and Tigerhair, their components and when and how to use if they are very different to the type you have already guided us on? I tend to use Allmetal as the first layer over any welds... good, bad, indifferent? Thanks!
    osage orange, K13 and loudbang like this.
  19. lostone
    Joined: Oct 13, 2013
    Posts: 1,297

    from kansas

    Back when I managed a body shop we used IR lights in the winter when it got really cold just to insure that the bondo was indeed setting up properly just to head off any problems that might arise.

    Worked well too, forgot which company we got the info from.
    K13 likes this.
  20. bill gruendeman
    Joined: Jun 18, 2019
    Posts: 333

    bill gruendeman

    Jim I do like sending time in AZ in the winter. I have heat but I don’t like to keep the heat on when I not going to be in the shop all week.
  21. Barn Hunter
    Joined: Feb 15, 2012
    Posts: 1,351

    Barn Hunter

    What should be used on fiberglass like this? Fillers and primers....Specifics would be welcome. Thanks! 20200112_161130 (2).jpg
  22. Paint Guru
    Joined: Sep 9, 2015
    Posts: 522

    Paint Guru
    from Bowdon, GA

    Awesome post! I need this wrote up as a handout for our shops!

    Sent from my SM-G955U using The H.A.M.B. mobile app
    Dino 64 and K13 like this.
  23. Gearhead Graphics
    Joined: Oct 4, 2008
    Posts: 3,534

    Gearhead Graphics
    from Denver Co

    Great post! Appreciate knowing the why and how of it as well. I'll save this in the brain
    BJR and K13 like this.
  24. Thanks guys glad you find it helpful.

    These products are known as reinforced fillers. (Allmetal/Metal2Metal are slightly different so I will cover those after)They substitute the talc and microspheres with some other type of fill material to product something that is stronger. They also can be used in areas where corrosion is a concern as they will not absorb water like a talc based filler will. They are classified as heavyweight fillers because they lack the mircospheres. There a basically 4 types of fill material used: long strand fiber glass fibers, short strand fiberglass fibers, and either fiber pulp or Kevlar pulp. These materials can be used individually or in combination to produce different products. They use the same BPO style cream hardeners and mix ratios as regular body fillers.

    Long strand fillers will be the strongest and best able to bridge gaps, holes etc due to the length of the fibers. They would be used when the most strength is required. They are not the easiest to work with as they are thick and don't sand well but short of using resin with mat or cloth they are your best bet if strength is required. They can also lose strength when used in thin applications as the fibers get push out and you are left with mostly resin. The fibers look like this:

    Short strand fillers are essentially the same but use a shorter strand of fiber which gives them less strength and less ability to bridge. They are however easier to work with as like with different sized talc they move around better and you are sanding a smaller strand. So these would be used where you still want some strength. Short strand tend to be the most popular reinforced products as they are a good compromise between strength and work-ability. There are also some short strand based products that are mixed with filler to make them even easier to work with but they will not be as strong and will absorb water. The fibers look like this:

    Finally there are multi strand or compound products. They mix the above two types of strand with some sort of pulp either fiber or Kevlar. These types of products are kind of the best of all world as the different sized fibers will kind of spread themselves out as you apply and give you strength regardless of how thinly you spread them. The biggest drawback to these is usually cost. The pulp fibers look like this:
    Fiber Pulp:
    Kevlar Pulp:

    Allmetal, Metal2Metal are kind of a unique category as they use a different style of resin that requires the use of a liquid MEKP type hardener. This is the type of hardener used with stand alone polyester resins. BPO and MEKP type hardeners are NOT interchangeable. They may seem to cure the product if used in products not designed for them but it will not be a full cure and can lead to failures down the road. Basically these products are resin and aluminum powder. They were developed as a replacement for factory lead when the use of lead started being frowned upon. Because the hardener is a liquid the above mixing method will not work and you will need to count drops and or use a scale for proper mix ratios. The ratios are still 2% weight by volume. These products don't sand well but do create a good hard surface. Aluminum powder looks like this:

    Something I should have mentioned in the filler discussion and I will go back and add it is not ok to use regular fiberglass resins to thin fillers and putties. As mentioned above they use different types of hardeners and adding resin to filler will almost always result in shrinking due to an incomplete catalyzation.
    Last edited: Jan 13, 2020
    williebill, lewk, flynbrian48 and 5 others like this.
  25. Good stuff, thanks
    K13 likes this.
  26. Any polyester based products, fillers or primers, work really well over fiberglass. If you would like to send me a conversation about what it is and what you are trying to accomplish I can go into more details.
    sleepchamber likes this.
  27. Fortunateson
    Joined: Apr 30, 2012
    Posts: 3,240


    K13, thanks for another educational tutorial!
  28. Excellent! Thanks for the useful info!!

    Sent from my iPhone using H.A.M.B.
    rust runner and K13 like this.
  29. 34Larry
    Joined: Apr 25, 2011
    Posts: 1,272


    Excellent info and instructions,. Thanks K13.:D
    K13 likes this.
  30. So in the 4” circle with a target of 4” ribbon you can have 3.6” to 4.4” ribbon of Hardener.

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