Sanding Foam for fiberglass cores

Can you suggest a source for sand-able foam core for glass layups ? Styrofoam is too flakey. I have lastafoam and other laminating sheets, but I need blocks to cut and shape into the teardrop aft of the Sam James cheek cowl for RV3/RV4.

Larry DeCamp said:

Can you suggest a source for sand-able foam core for glass layups ? Styrofoam is too flakey. I have lastafoam and other laminating sheets, but I need blocks to cut and shape into the teardrop aft of the Sam James cheek cowl for RV3/RV4.

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1-2"x4'x8' dense insulation board from the local box store. Cut out /laminate together layers to your specific needs and sand away.

Simple example from DanH:
http://www.vansairforce.com/community/showpost.php?p=698219&postcount=69

Here's a more in depth one.
http://www.vansairforce.com/community/showthread.php?t=43391&highlight=DanH+mold

Much more info in the Fiberglass section.
http://www.vansairforce.com/community/forumdisplay.php?s=&daysprune=&f=106

The composite builders will tell you urethane foam is the easiest to shape. That foam was a greenish color or maybe tan when I was building the LongEZ. Readily available via A Spruce or you local flower shop! There are several densities for that, not sure it matters for your use.

go here:

https://www.fibreglast.com

Thanks much to all !

I have 1? pink from building the house. It does not sand like lastafom and blue foam from ACS. I will check big box for 2? before proceeding. Thanks Larry

https://www.aircraftspruce.com/catalog/cmpages/lastafoam.php
this should sand well tech name is Polyurethane foam , called Last a foam from AS

I used the 1" pink foam to make a couple of plugs. When I covered it with bondo to make a slick surface, the foam melted. There was much sadness at the Boatright house that day.

Moral of the story: For direct contact with the pink foam, you don't want to use polyester based filler or resin.

Pink foam will sand just fine with 40 or 80 grit. Then as it is close to shape 100 or finer will leave a very nice surface. Plaster works best for finishing, but Dans method of a final coat of something, epoxy or something will work best if you want the part to separate cleanly and reuse the mold. I don't like drywall compound as it takes too long to dry. If sanded smooth enough not much is needed. Don't forget the PVA.

Kyle Boatright said:

I used the 1" pink foam to make a couple of plugs. When I covered it with bondo to make a slick surface, the foam melted. There was much sadness at the Boatright house that day.

Moral of the story: For direct contact with the pink foam, you don't want to use polyester based filler or resin.

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Microballoon as a skim coat works great over the pink insulation foam. I glue pieces together to get thickness using 3M spray glue and it works great. The foam itself sands nicely and the micro is easy to sand as well.

Capflyer said:

Microballoon as a skim coat works great over the pink insulation foam. I glue pieces together to get thickness using 3M spray glue and it works great. The foam itself sands nicely and the micro is easy to sand as well.

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No doubt.

The reason I was using bondo was that it cures and is ready to sand in minutes, not hours. If I'd known, I could have probably done my fill work with spackle, which would have dried in an hour or two and would have been fine for my application.

Use of Bondo

You can use Bondo on the Home Depot pink foam, just paint on a coat of epoxy before using the Bondo.

My typical plug is built up of 2" pink foam held together by dabs of 5 min epoxy. Make sure the dabs of epoxy are not where you will want to sand as they will be hard spots. Shape with 36-80 sand paper and finish with fine 150-240 to a smooth surface. I try to use the factory finish for any large flat areas. If there is any heavy removal required that I can't just cut away on the band saw, I use a 4 1/2" grinder with a 36 flap wheel for the heavy work, and a shooting board for the fine sanding.. Do not use paper in the hand for anything but small 3D inside or outside curves. You will end up with lots of low spots.

Paint on a coat of epoxy (I prefer a fast cure formulation for this) to protect the foam from the Bondo, then Bondo away. Bondo starts to kick in about 5-10 min so you have to work fast in smaller areas. Sandable in about 30 min (or when it doesn't blind the sand paper). Wax, PVA (or packing tape for simple shapes) and ready to lay glass.

This thread is starting to bother me. Having built a fiberglass airplane, some of the techniques mentioned here are just flat out misguided. First off, you're building an aircraft, not a boat where weight is not a concern. You want your fiberglass work to be light. Bondo is heavy...Micro is light. Foam from Home Depot is heavy...aircraft grade foam is light. Just ask Burt Rutan. Secondly, do you really want to save $5 on your $100,000 aircraft by using some unknown (and heavy) construction foam from Home Depot rather than the light weight polyurethane foam readily available from Aircraft Spruce or other places? With the salmon-colored 2 lb./ cu. ft. foam from ACS, you can cut it with a bread knife and finish sand it by using a block of the same foam to work it. (Wear a mask!) ACS also sells more dense Last-a-Foam for applications needing more compressive strength. (Sadly, the Clark Foam is no longer available.) Thirdly, when you're sticking two blocks of foam together, the adhesive is going to leave a hard glue line at the joint which will be impossible to sand out. (Disclaimer: I've never tried the Scotch 77 spray adhesive, so that might avoid that problem). My advice is to spend an hour to watch the Burt Rutan video linked above to learn the basics of fiberglass construction.

John, I don't think anyone is talking about structural core, intended to remain in flight parts. A fella could make throwaway forms from a stack of stale Rice Krispie treats. However, pink OC foam works better.

FWIW, it's actually less than 2 lb density.

Key here is price and availability. A 2" thick x 48 x 96 OC board is $30, delivered to your local HD, or about $5.60 per cubic foot. 2 lb density poly foam at ACS would be $125 for the same sheet, plus shipping.

I think there is a big difference in using foam for a structural core or just using foam for a shape allowing you to lay up several layers of glass for as plentium. Depending on the resin and foam there are some that will eat foam cores unless you use a duct tape, bondo layer or seran wrap for separation. After all, on a plentium all you want is to make a shape and then remove all the foam core used for that shape. Just use you homebuilding skills as best possible and sometime it will take a couple of tries!

DennisRhodes said:

I think there is a big difference in using foam for a structural core or just using foam for a shape allowing you to lay up several layers of glass for as plentium.

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That's entirely correct. Sorry, I missed that part of the original post. For molds I've used all kinds of stuff - Styrofoam, hot glue, Bondo, duct tape, etc. Please disregard my previous rant.

snopercod said:

That's entirely correct. Sorry, I missed that part of the original post. For molds I've used all kinds of stuff - Styrofoam, hot glue, Bondo, duct tape, etc. Please disregard my previous rant.

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Ten lashes with a wet noodle for you!

I'm going to my room now...

Lessons learned today, thanks to all.

FWIW if you, like me, don't already have a system that works for you.
1- Pink "big box" foam works fine. Some is blue, some green, but about the same.
2- 3M Super 77 spray adhesive laminates layers fast, easy and well.
3- A fine tooth serrated bread carving knife works great. The electric Turkey carver might be the ultimate convenience.
4- a Vixen flat file does a nice fairly smooth job of refining the cuts.
5- Of course, 40 to 80 grit paper on a flat surface finishes good enough for epoxy sealant.
Happy fiberglassing

This is going to sound weird, but a great tool for the blue or pink home depot foam is a file card. It is a little course until it gets foam packed in the bristles, but after that it works really well. More aggressive than sandpaper, so good for shaping if not finishing.