I have 2 questions for any one who can help,

1- My building instructions require that I mix epoxy with Microbaloons to make a fillet , no mention of how much or in what ratio ? Why do you need microbaloons ?

2- When thinning epoxy to brush a firewall or inside of a cowl can I mix in some black paint to give the mixture the colour I need ? if yes will any paint do ? Or must one paint first and add the brushed epoxy later. Thank you.

1. I wonder if the instructions really require this, or is this only how the manufacture did it. If this structure has some real strength needs then the epoxy/microballons is a good way to go. If it doesn't require strength, I would tend to use one of the easily sandable fillers, similar to spackling compound. I've never found epoxy and microballons to be easy to work with and simply don't use it. Others might like it and can give you tips on it's use, in which case I'd be interested in hearing about, also.

2. Paint color first, let dry and then paint on the epoxy. Since there are so many different paints, what will mix with epoxy and work would take experimentation. Maybe someone else knows of a combination which will work. I thin epoxy with rubbing alcohol. This does slow the cure rate down measuribly. In the engine compartment, thinning isn't really needed. Use slow cure epoxy and paint on with a small disposable brush.

Yeah! What he said. I found epoxy and micro-balloons to be real hard to sand. Light weight spackling works really good in most cases and sands easy. When you need more strength, finish reson and micro-balloons works pretty good. Sands like hard balsa. I mix it up a to a consistancy between honey and tooth paste. Depends on where I need it. Thicker if it might run or droop.
Edwin

I do not find the epoxy/microballoon mix difficult to either apply or to sand. Just use a slow epoxy, thirty minute or slower. First thouroughly mix the epoxy before adding the micro balloons. Add balloons, stir well, add more, stir some more etc. untill you have paste about as thick as heavy peanut butter. It will hold its shape then and will be quite easy to sand. You can feather the edges by dipping your finger (wear rubber bloves if you are alergic to epoxy) into alcohol and rubbing the edges to smooth and feather them. Let it cure well before sanding.

Thanks for the advice.

I mix 1 part microballons to 3 parts BONDO (yes Bondo). Mix the microballons THOROUGHLY into the Bondo BEFORE adding the hardener. The end result is relatively light. It has substantially more strength than a light weight filler (less edge chipping and cracking ) and is fairly easy to sand. It is going to sand "harder" than a light weight filler, but no where near what straight Bondo would be like.

If you decide to go this route (or use the SIG Epoxy-Lite ) have some 91% rubbing alcohol handy and smoothe the surface of the Bondo while it is still wet. The alcohol will allow you to shape the fillet easilyand the Bondo will not stick to whatever you are using to shape it. Make sure you let the Bondo dry overnite before doing any sanding. The Bondo may APPEAR cured, but it isn't. It will take several hours for the alcohol to fully evaporate from the surface (and the little bit it will soak into the Bondo ).

For your engine compartment and fuel tank compartment - thin the epoxy with either acetone OR DENATURED alcohol. Thin about 10% by volume. Regular rubbing alcohol contains water and epoxy and water do not mix.

Hope this helps.

Rubbing alcohol worked fine for me, even if there is water. However, DNA might work better so next time I need to thin epoxy, I'll try it! Thanks Campy

The thing to remember when adding microballoons to either epoxy or polyester resins is this, the more balloons you add, the softer the mix will be and hence easier to sand. I find that the polyester resin and microballoons mix is easier to sand to a feather edge than the epoxy mix. Also, when the polyester starts to kickoff, the mix gets to a consistency like a cheddar cheese and is easily trimmed with an xacto knife or even coarse sand paper where it actually starts to roll off, just be careful at that stage. Also, when applying, make sure you pat it in to make a solid contact.

I often wondered how well balloons and bondo would be together, just a lighter bondo I always thought. I mean bondo is basically just polyester resin and talc. Glad to know someone actually tried it and it works.

The biggest thing to me is, after all the years I've been in this hobby, I have found few times when I absolutely needed to use the epoxy or polyester resin and Micro balloons mix. As always, there are several ways to skin a cat.

I've found that when using epoxy and microballoons as a filler, after its in place, use a piece of wax paper over it to smooth and feather. Leave the wax paper in place until the epoxy is fully cured (8 hours) then just peel it off. It will leave a nice smooth surface that should very closely match your final profile. Really reduces the amount of sanding required.

Brad

Saran Wrap works better than wax paper to have a smooth surface and has less tendency to stick. I often use it when fiberglassing a wing's center section together as it leaves an absolutly smooth surface requireing no sanding and perfect feathered edges.

I like the wax paper because its stiffer than Saran Wrap, and I can easily use a credit card on the top to work the epoxy and microballoons. The Saran Wrap tends to stick to the credit card.

To each his own.

Brad