what seems like about 2 years behind in the shop my patience isn't what it
could be these days.
So with tank "pre-prep" on the top of my mind, after spending an entire day
getting parts prepped for sealer tomorrow, I offer the following in addition
to the great advise Ken has already offered and like Bob says most of this
has been covered for years and can be found somewhere in the archives.
I see leaking tank after leaking tank that the builder put ABSOLUTELY no
effort or time into preparing to build a leak proof tank. I regularily find
no evidence of scotchbriting a nice cross hatch pattern in all the mating
parts, if any scotchbriting at all. Guys expecting
Proseal/Chempac/PRC/Flamemaster to stick over top of epoxy chromate in the
tank area. Now that just ain't gonna happen!
To give an idea of time... I spent 4 hours scotchbriting the 8 ribs, 6 rear
bulkheads and all the little filler clips and fuel fitting bushings for two
tanks today for a pair of Rebel wings. I had already previously spent a few
hours scotchbriting the main spar and stringers where they go thru the
outboard tank rib. This was all done with a 2" coarse scotchbrite disc in my
air drill... then everything was gone over with a piece of hand held coarse
scotchbrite material to be certain that everything was done and to get in
all the little tight corners/nooks and crannies! Every stringer
"clip/filler".. end rib angle filler.. spar filler.. etc scotchbrited by
hand on both side.
Prior to scotchbriting EVERY inch of aluminum was precleaned with Acetone.
After scotchbriting I spent about 2.5 hours cleaning everything with Acetone
again... like Ken says... you clean until the cloth stays white and then
some. I use white papertowel.. and take a single sheet and fold it numerous
times. This way you have a "sponge" holding Acetone and you can refold it in
numerous directions before throwing it in the fire once you have every face
of the thing black.
During cleaning I even caught myself where I had missed a joggle on a rear
bulkhead with the scotchbrite wheel.... thank god as that is the most prone
place for leaks. You need to scotchbrite the hell out of the joggle area on
the bulkhead and then do the same on the inside surfaces of the rib flange
where it fits in place. This joggle area MUST be sealed while you are
riveting in the tank bulkheads or you are doomed from the start! Be sure to
scotchbrite the underside of all the flanges in the tank area.. and ANYWHERE
that sealer may sit be it from what you spread or from where the dipped
rivet gets pulled tight.
Now keep in mind.. the time frame above.. this is strictly the ribs,
bulkheads and clips. I haven't even mfg'd the tank skins yet as there is no
point at this stage as the hole pattern will change slightly once I seal and
rivet the rear tank bulkheads in place. So tomorrow the ribs/bulkheads get
sealed and riveted hopefully... and then after a few days cure I'll be able
to start making skins... and more scotchbrite.. more cleaning.. etc.
The trick is to work your way towards a complete "bathtub"... getting the
bottom skin/ribs/bulkheads all in place and sealed. NO .. more sealer isn't
the answer, good prep on the mating seams with about an extra 1/2"
scotchbrited out onto skins and ribs is the key to make one nice smooth
light coat of sealer do it's job properly by giving it something to adhere
to.
When it comes time for the top skin.. there are two ways to do it... and you
need to preplan this so your stringer filler "clips" have been fitted inside
or outside the tank.
If your top wing skin is already in place with stringers on it..... before
you can place the top tank skin there should be no filleting required
anywhere in the tanks. All that should be left to do is put a bead of sealer
around the perimiter and down the stringers and place the top skin. Pre
cleco the holes in the tank skin before placing..... a pair for every rib
flange.... a pair in every bulhead flange and a few for stringers.
Now the big trick to being able to do what I've stated above.. is you have
to have your stringers sealed thru the outboard rib... and of course it's
waving all over the place so how the hell do you do that and have it in the
correct place... right. Well you make little tying clips from a hole mess of
little rectangles of .025 or .032 scraps. Transfer the hole pattern from
your tank skin overlap to wing skin holes, to the outboard fuel tank rib
flange holes. Then cleco these little "tie clips" between the wing skin and
the outboard tank rib flange. While you are at it take another long strip of
material and transfer the holes on the tank skin between the three
stringers.. a front spar hole and a rear bulkhead hole. Now the clips will
hold the rib exactly where it's supposed to be and the longer strap will
hold the stringers fore/aft where they are supposed to be.... and you can
install your stringer fillers now with sealer and rivets. Then you can work
the sealant in.. around.. thru.. the stringers and both inboard and outboard
of the tank until you are absolutely sure there isn't a pinhole thru there.
Once you've got that covered then you want to build dams between rib flanges
and stringers and at any corners of rib to spar or rib to rear bulkhead
junction until you have a complete.. flat.. sealing surface for the top skin
to set down on.
If you haven't placed your top wing skin yet or riveted the stringers to
it... you can do your tank first. Follow most of the above other than the
need for the little rib holding clips etc. Seal and rivet the stringers to
the top tank skin. Have the tank as ready as possible for a flat sealing
surface and do the cleco pairs etc.. put the bead of sealer down and get the
skin in place and start dipping/rolling rivets and placing them in holes.
Have a piece of scrap 0.025 material ready to slide in under the tank skin
at the main spar, to simulate the wing skin going between them, so sealer
doesn't ouzz into the space. Now pull all the rivets that you've
dipped/rolled in sealer before placing and once complete you can get in
under the tank skin overhang and work on filling the tank stringers/fillers
to a perfect seal.
When in doubt on the stringers.. turn the lights out in the shop and shine a
bright light down the stringer from the wing tip and look in the tank filler
for light coming in... or visa/versa.. light in the tank and go look in each
stringer curl at the wing tip for light shining out.
Enough for my head tonight after all that Acetone...
;O)
Wayne
----- Original Message -----
From: "Bob Patterson" <bobp@prosumers.ca>
To: <rebel-builders@dcsol.com>
Sent: Friday, January 06, 2012 4:49 PM
Subject: Re: [rebel-builders] Re: [rebel-builders] top tank skin removal
Also - use separate clecos that are stored in acetone to
avoid oil causing leaky rivets.
Lots of detailed instructions in the Archives ! Look for
emails from Wayne !!
......bobp
On Friday 06 January 2012 16:10:08 Ken wrote:Well yes but some of the changes I made were:
1. Use tank skins that are larger than the tank so that the joint of
tank skin to wing skin is entirely outside the sealed tank. Why put an
extra layer of aluminum in the sealed joint and make more leakage paths.
A second benefit of this is that you can build the tank and recoat all
external edges aft of the main spar before riveting the top wing skin on.
2. I cleaned with acetone, then scotchbrited, then cleaned again with
acetone to remove the dust. That is different than my manual specifies.
If the rag is black then keep cleaning. Some guys even soak the rivets
to make sure they don't have any oils on them.
3. As per previous post, re-coat with brushable proseal through a large
oval botttom access hole when building. It is very easy to seal an
access hole on a flat surface.
4. Use a weigh scale when mixing the proseal. My sources said not to
thin proseal. Other sources say it is OK but nevertheless I purchased
the brushable product for recoating purposes.
5. Pull rivets by hand or with minimum air pressure to give the proseal
a chance to squeeze out. The rivets should pull bit tighter if pulled
slowly I think.
6. If required, immediately after pulling all the rivets invert the tank
so proseal tends to spread out and make a natural filet.
On 06/01/2012 2:17 PM, Doug Farrow wrote:Is there a way to do it so there are no leaks from new.
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