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Dollar Tree Foam Board Waterproofing

ofiesens2

Professional noob
#1
I watched the Flite Test Snowball episode and wondered why DTFB is not water resistant. My question is, is it the paper backing that doesn't like the water, or is it the actual foam, or both?
 
#3
is it the paper backing that doesn't like the water, or is it the actual foam, or both?
It's the paper. Paper already has quite a bit of water in it (approximately 5% to 20% depending on the exact type) and it likes company. The cellulose fibers act like little hydroscopic sponges, absorbing moisture (including from the atmosphere).
 

ofiesens2

Professional noob
#4
So technically I could build a waterproof plane just by taking the paper off the foam?(and fixing the issues with the bevels on the control surfaces)
 
#5
The cells in the foam will absorb a certain amount of water (but I don't think it would soak in). On the other hand, the paper gives the foamboard much of its rigidity. Cut a strip of DTFB about 1" wide by 18" long, peel the paper off both sides, and you'll see what I mean. :cool:
 

rcspaceflight

creator of virtual planes
#6
You certainly could remove the paper to make it water proof, but it's an extremely good idea to replace the paper with something. You'll lose a lot of strength when you remove the paper. Packing tape works as a really good replacement.
 

bamjoop

Junior Member
#7
#8
In addition to the above, it is important to note that there is no adhesive holding the paper onto Dollar Tree foam. This is both a blessing (for when you want to remove the paper) and a curse (when you don't).

Once the moisture content of the paper reaches a certain point, it will easily de-laminate from the foam, and this will cause a *dramatic* reduction in strength and stiffness.

You can use fiberglass + resin on foam, but this can add a huge amount of weight if you are not very careful. Theoretically you can replace the paper with 3/4 oz cloth if you can manage to use a minimum amount of epoxy. You *must* use epoxy, btw, as polyester resin will dissolve foam.

If you want a good combination of strength, durability, low weight, low cost, and ease of use, you can strip the paper that comes with the DT foam and replace it with tissue paper bonded to the foam with *water* based poly-urethane. The WBPU will both bond the tissue paper to the foam and seal it from moisture.
 

jhitesma

Some guy in the desert
Mentor
#9
If you want a good combination of strength, durability, low weight, low cost, and ease of use, you can strip the paper that comes with the DT foam and replace it with tissue paper bonded to the foam with *water* based poly-urethane. The WBPU will both bond the tissue paper to the foam and seal it from moisture.
Hadn't heard of that before, may have to give it a try. The only thing I was disappointed about with minwax was that it doesn't really help bond the paper to the foam any better and I was really hoping it would.
 
#10
I was thinking one of the flite test videos instructed not to use the waterbase poly? But that was being applied over the stock paper on DTFB, not tissue. Im surprised to hear the application of poly does not help the paper adhere to the foam. I thought it would soak thru and make a better stock paper to foam bond, but I have not personally tired it. I have not heard anyone mention using the minwax poly in the spray can verses brushed on either, but thought this may be an option.
 
#11
You are correct Outlaw, you have to use the oil based version. I've done the brush on Minwax method on several planes now, and I'm happy with it. Not only does it provide water resistance but it makes the surface great for painting. I've painted with both rattle cans and airbrush. I'm not the best painter, so I like not having to do a bunch of light coats. I do one light coat and then a finish coat. I've been brushing it on with those throw away foam brushes. Sometimes just leaving it to dry without even getting the excess off with a paper towel.
 

Craftydan

Hostage Taker of Quads
Staff member
Moderator
Mentor
#12
the problem with the water based poly is that the paper warps as the water soakes into the fibers and the shear stress delaminates the paper from the board. The oil based poly coats the fibers instead sealing them against water intrusion and the warping that follows.

As a side effect the oil based poly makes a great primer for painting with water or oil based paints, since the cured surface will take paint well and the fibers will no longer warp from wetting.

If you strip the paper off, the water based poly makes a great surface protector, primer or laminating glue, but in lamination it needs to be pressed while drying which will take about a day or so. It's good for building up thcker sheets, or even blocks of foam, for sanded wings or fuselage elements.
 
#13
Outlaw: You must read threads in context. Water-based polyurethane is used to bond a lightweight paper to foam *after* you have already removed the factory paper. This technique is used to eliminate the de-bonding issues. Waterproofing is merely a bonus.

This technique is completely different in every way than the paint-on, wipe-off technique that uses oil-based polyurethane.
 
#14
3/2/14 Tried some foam board $ 2.50 from a national chain craft store (Hobby Lobby) ,a type I hadn't seen before and found out the paper is too tough to make a paper hinge as usual. But the paper is much more water resistant and tolerated hosing it down with spray paint . The core seems to be Depron. This is not their $5 a sheet stock but seems to be their economy line and comes in larger sizes.
 
#15
Made the paper brittle and tended to delaminate faster ,you would have to seal the edges with tape or hot glue as suggested by josh a few weeks ago in tips.. to get real longevity in your build.
 
#17
First off I tired several test to keep my paper peeling after being painted,

The final Result was building a OV 10 Bronco of
Adams Foam Board unpeeled
Blue Foam Board unpeeled
Adams board peeled
Blue foam peeled

I applied Minwax Half-Pint Fast-Drying Satin Polyurethane Oil Based with foam brush, let it sit for several hours then wiped down both sides.
Laid out the plans and cut and build.
For lamination I use Gorilla glue
For sealing edges I use hot glue and wiped with foam scrap, wiping of excess
Airbrushed, spray painted and hand painted, seeing there was a difference.
Last I applied a coat of Ultra Cover 2x ( rust-oleum) Clear Satin.
I then hung the plane outdoors for 1 months, the plane went thur rain, sleet, snow, frooze, and thawed several times
here are the redults.
No peeling, not seperation, but blue core foam if very soft and dents easy. When it fell down paint did come off, last coat was very light, first time using Ultra Cover.
I bought everything at Lowes, they offer 10% discount for Active Duty and Veterans!
 
#18
I use a monokote heat gun and suspend a sheet of foam between to flat surfaces and place a little weight in the center and heat the foam and with a little practice it comes out straight ...if it is twisted place the to lower corners lifted off the flat surfaces and use the same process applying heat to get the twist warp out...With a little practice and patience you can get the sheets back nice and flat the heat also helps take out the ripples in the paper if present...
 

bhursey

The Geeky Pilot
#20
I used minwax clear polyurethane. it worked very well after I could literally put the plane under a faucet and it would just bead right off. Not it will give your planes a tad off white color. Very minor