IN PRAISE OF WATER-BASED POLYURETHANE VARNISH
by Ironsides

What a mouth full!  From now on the substance will be known as WBPV.  So what is it?

WBPV has been around for a while, it is just becoming more popular with modellers.  It is used in homes to varnish wooden floors.  Its most usual application in model building is to use it as a substitute for epoxy finishing resin when making fibreglass over balsa over foam structures.  The WBPV has six main advantages of the epoxy resins:

                1.    costs about one third
                2.    has no odour - epoxy stinks - your spouse will love WBPV
                3.    sets up faster - epoxy is slow
                4.    weighs less - the water evaporates
                5.    less harmful - epoxy vapours can be nasty
                6.    readily available at major hardware outlets.

Please note that all WBPV does not react the same way.  The experiments to date were all done with INTERIOR - CLEAR varnish.  It has been noted on the Internet that WBPV with colour pigment tends to yellow over time.  So does interior clear WBPV when exposed to sunlight.  The market provides exterior clear WBPV that has UV filters in it to prevent yellowing from sunlight.  All the discussion hereafter concentrates on the less-expensive clear interior WBPV.

Here is how clear interior WBPV is used in a typical construction of foam core floats.  Once the balsa sheet is on the foam core and has been sanded and tacked to get the dust off the fun begins.  Cut out the piece of fibreglass cloth (typically half ounce weight) a bit oversize to the side/bottom being covered.  Paint on WBPV with a SYNTHETIC bristle paint brush.  [Those expensive NATURAL bristles will cause bubbles].  Let the WBPV dry until tacky.  With the help of a friend, lower the fibreglass onto the tacky balsa.  Paint on a thin layer of WBPV to smooth out the fibreglass.  Let it dry completely - normally about 4 hours - see the difference already.  Paint on a second coat of WBPV and let it dry - this time it will only take about an hour - wow, what a difference over epoxy!

Once dry, lightly sand the edges of the cloth to get rid of the excess material.  Continue the process until all surfaces are covered.

CAUTION:  On the News Group rec.models.rc.air, a user reported that the WBPV caused the balsa  to swell up.  He guessed that this was due to using too much varnish for the first coat which, due to its water base, got into the balsa and caused it to swell.   It is suggested that the first coat be very light - perhaps even rubbed on with a cloth dampened with WBPV rather than risking the liquid loading that a brush can deliver.  Equally, you could cheat and use a light spray of

Now we have to make a major decision.  Are we going to want the natural weave of the cloth to show through, or do we want to fill the weave for a nice finish?  If the first option prevails, then lightly sand the cloth with 300 weight wet paper.   The painting phase will be covered below. .

If you want to get a very smooth finish, then we have the option of two filling techniques.  Both involve the addition of powder to the WBPV.  For a long time talcum powder has been the most common, but corn starch becomes an option because the varnish is water-based.

Using talcum powder, mix a solution of 20% powder and 80% varnish.  Paint on to the weave and let dry - it won't take long.  Once dry, wet sand with 300 grade.  Do it all again until you are satisfied.

Using corn starch, you might find it easier to pre-dilute the starch about 50/50 with water and then add it to 100% WBPV.  Paint and sand as above until you are happy.

CAUTION:  Whether you use talcum or corn starch, make sure you put on one last coat of 100% varnish.  WHY?  Because the solvent in some spray paints seems to "cook" powder and give your smooth finish an "acne" look.

Once the final coat is on, wet sand with 600 grade paper and paint in the normal manner.  WBPV does not seem to need any primer as long as it is roughed up slightly.

Experimental Stuff

Our group has been experimenting with another use of WBPV.  That is, we want to cover both balsa and open spaces, such as wings with the traditional "silk and dope".  Essentially, we have subsituted WBPV for dope and SIG Koverall ® for silk.

We have discovered that you need to be aware of that INTERIOR CLEAR seems to give much better adhesion characterists that EXTERIOR CLEAR.  Without proof, it is guessed that the UV filters added to the EXTERIOR version lessens the adhesion characterists.

We paint the balsa with INTERIOR CLEAR WBPV on the outside edges of the area to be covered, then let it dry.  We cut a piece of SIG Koverall® slightly oversize and put in on the balsa piece.  Then we take our hot covering iron and iron on the Koverall over the dried WBPV.  We think the heat reactivates the polymer in the WBPV and glues the Koverall to the balsa.  Then we paint another narrow coat of WBPV onto the edges of Koverall to match the original WBPV pattern on the balsa.  This second wet coat bonds through the Koverall to the original dried coat and cements the Koverall in place.  We let the second coat dry.

We shrink the Koverall in two ways.  We use a hot iron or we use a hot air gun.  We are careful in both instances to stay away from the edges where we used WBPV as glue - it releases and the Koverall pulls away.  With a hot air gun, the easiest thing to do is mask the "glue joints" with a metal shield.

Once the Koverall has tightened, we fill the weave as outlined before and paint with WBPV.

Others have found that superior adhesion can be achieved by dressing the raw balsa edges with Weldbond ®, letting it dry and then using a covering iron to reheat and achieve adhesion to the Koverall. Subsequently, the WBPV is used to fill the weave as outlined above.

Since this is still an experiment, we suggest you try a test piece before commiting to that brand new model you spent all winter constructing!
 

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