FW: REFLECTOR: Scratch Test

[Phil Hooper]

Gang, Andy Millen sent this very good answer to me regarding my question
about the scratch test.  With his permission I am posting his reply to the
Reflector asking for any additional comments from your collective wisdom and
experience.

Of particular interest:  the "drill test" recommended by the factory, and
use of denatured alcohol to restore old hardener to a proper consistency.

I'll appreciate any added thoughts. 

Here is Andy's treatise (with my original post at the end):

--

Good questions.  Don't apologize for asking this type.

This has been my experience.

I have never had the hardener crystallize on me.  However, this is said to
be quite normal and heating it to reliquify is the correct course of action.

The scratch test.  Let it cure in the pot for 24 hours.  Take a nail or
screw and attempt to scratch the surface.  If it scratches, then it cured
properly.  This will show you if it is still rubbery.  A bad mix may never
cure to hard.

The biggest problem with the hardener is the styrene evaporating.  When it
gets to a low level, the hardener's characteristics will change.  It will
get thicker.  The epoxy will cure much faster (complete cure in less than
eight hours).  Worst is the bond between the new glass and the old will be
very weak.  It may pass the scratch test, but is still not acceptable.

I called the factory and they gave me the "drill test."  (Unofficial Name)
After the glass has cured, drill a small hole through the lay-up.  If it
goes through clean you have a good lay-up.  If you see a clear delamination
around the hole, noted by a large whiteish area, then you have a problem.
The good and bad is that the lay-up will delaminate easily so you can get it
off.  You might need a little help from a heat stripper gun.

My kit came with the typical 5 gallon bucket of hardener.  It was good for
about the first year and a half and then went bad.  It was a frustrating
shame as there was better than two gallons still left in the bucket.  Many
people have suggested to Velocity that they should ship the kit with a one
gallon can and then provide the rest on request as the builder needs it.

I have never heard of using denatured alcohol.  I have heard of other
builders using Acetone.  I do not.  That being said, here is my thinking:

1) I have never had a problem with the epoxy saturating the cloth, so I have
never felt there was a need.  I keep the resin in an epoxy warming cabinet.
I DO NOT keep the hardener in there as I feel the heat promotes styrene
evaporation.  The warm epoxy flows very well.

2) I believe the hope/theory is that the thinning agent will evaporate out
of the lay-up leaving nothing but epoxy.  I don't know that it would ALL
evaporate as I can believe that some could be trapped.  Alcohol and Acetone
are decidedly NOT structural elements.  If it does all evaporate, it has to
leave a void somewhere possibly at the most minute levels.  I would be
concerned for the strength of the aircraft.  I have not heard of Velocities
or other composite aircraft falling from the sky due to mixing in thinners,
so this is MY thinking only.  I just don't want to take the risk.

3) I do not know that the chemical and structural properties of the epoxy
are unaffected.

4) Dow could have made the resin or hardener thinner if they had felt it
necessary.  If it is too thick to spread, it is probably bad and needs to be
replaced.

44/100 is precise, but not difficult.  I use a gram scale and can mix
batches nicely in the right ratio.  I put the resin in a gallon jug with a
pump.  These can be bought at most food service stores.  I put the hardener
in an old pancake syrup bottle.  This makes it very easy to control.

The resin should last forever.  If it crystallizes just warm it up.

FWIW,

Andy


-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Phil Hooper
Sent: Wednesday, July 06, 2005 11:51 PM
To: 'Velocity Aircraft Owners and Builders list'
Subject: REFLECTOR: Scratch Test


Collective.
 
Can someone explain how to conduct and evaluate the scratch test?  My
hardener crystallized, and I'm in Southern California.go figure.  It's only
9 months old.  I placed the jug in hot tap water and it went back to
solution with no problem.  I have done a small lay-up with three layers of
bid, and left some in "the pot".  In 24 hours, how would I check it...see if
it scratches with 36, dig at it? 
It has been suggested that older hardener can be diluted with denatured
alcohol.  How do you evaluate how much is enough?  Is it sound practice?
When it gets to the stage that it needs dilution, I wonder whether it would
be worth a trip to Spruce.15 minutes from here.  44/100 sounds pretty
precise to me. 
It seems like the main resin will last a long time.  It is the hardner that
ages, and that the styrene vapors out of the bottle.  Is that correct? 
 
As you can surmise, I'm a slow builder.  And just for a little patriotism
after the 4th (and some pride), I've attached my excuse that lives 329 miles
to the north, born April 15.
 
BTW, sorry for the ignorance and thanks for the help.