Priming the fuselage

As soon as the fuselage is clean corrosion will begin so it is vital that the bare fuselage is painted as soon as possible to protect it from the atmosphere. I completed the cleaning one morning which left the rest of the day, about 7 hours, to prep for, and apply, one coat of primer on the fuselage. In total, the airframe will get three coats of paint:

1. An acid etch primer (green)
2. A 2 pack epoxy primer (white)
3. A top coat of white two-pack

I'm using an HVLP (High Volume Low Pressure) spraying system to apply the paints which I'm told is easier for the beginner to get good results with. The system is self contained using a turbine unit to provide air for both the spay gun and a half face mask, essential when working with harmful paints. Details of the equipment can be found at http://www.flexiblesanders.co.uk/proddetail.php?prod=COMBO4STAGE&cat=21 although I did upgrade the gun to a gravity feed type at the time of purchase.

Prior to spraying the fuselage was mounted onto a rotating jig (courtesy of Rob Lees again) which makes the painting process much easier.

I started by spraying all the inside tubes first and gradually working outwards to reduce the chance that I would smudge areas that had already been painted but because of the complexity of a fuselage I did manage to paint myself into a corner on a couple of occasions. I ended up alternating from front to back to allow painted areas to go off before returning to those tricky areas that had been previously missed.

Several tips here - get used to your gun, how to change things like the amount of paint sprayed i.e. how wet it goes on as this does vary with the type of paint your using. Practice spraying left and right handed as you're sure to need to change hands at some point. Don't let your gun run dry before recharging with more paint as the paint may dry in the gun nozzle causing problems with delivery. Always clean your gun after use.

By the end of the day I had a green fuselage and in an ideal world I would apply the next coat (white epoxy) immediately but alas I had run out of time.

The following weekend was spent checking that everything had been sprayed, all those tabs, 'u' shaped channels, weld clusters etc and where necessary those areas were cleaned ready for retouching. Now because the epoxy hadn't gone on straight away I had two choices for the next stage.

1. lightly scuff the surface to provide a key for the epoxy.
2. spray a light flash coat of etch prime on then apply the epoxy.

I decided on the latter which would ensure a good key over the whole frame and anyway a second light coat wouldn't add much weight at the end of the day. Two hours later I was ready to apply the epoxy using the same techniques as for the etch prime. If I had any black epoxy, it would have been worth mixing a little in so that when it came to spraying the white top coat, I would be spraying onto an off-white colour making it easier to see what I was doing - problem was I didn't have any.

The epoxy took about 5 hours to apply with a lot of time spent again on all the tricky corners, tabs, clusters etc. The two photo's show the internal tubes painted white and the final complete airframe which I'm quite happy with. The top coat could just be applied to the cockpit area which will be visible after the fuselage is complete but I may decide to cover the whole frame with top coat as it will give added protection for very little increase in weight.

In the mean time I have a lot of other bits that are ready for grit blasting and painting up....

Cleaning the fuselage

Christmas came and went, all the necessary welding work had been completed on the fuselage and so it was time to strip the fuselage frame of all it's paint. I had thought long and hard about how this should be done and had settled on grit blasting rather than paint stripping mainly because paint strippers are messy and toxic (in most cases). I had also decided to do the grit blasting myself rather than transport the fuselage to a professional because (a) I didn't have a means of transport and (b) because of the costs.

There were a number of prerequisites to the stripping process.

1. Create a booth that would take the whole fuselage and keep the rest of the workshop free of dust. This would also serve as the paint spray booth later on.


2. Make sure I had the appropriate blasting equipment including safety wear and consumables.


3. Get some paint stripper just in case.


4. Arrange for the AD inspection on the strut attachments at an early stage when the weld clusters had been cleaned up.

The booth was made from white tarpaulin sheet hung from the roof beams with the lights inside the booth. The dimensions were about 20 x 10 feet giving enough room to work around the fuselage. I also have a small extractor fan used to keep the dust down.

I bought a cheep hopper (http://www.northerntooluk.com/sandblasting/sandblasting-accessories/portable-shot-blasting-kit_146993E.html) which holds a 25 Kg bag of grit/bead and utilises my compressor running at about 115 psi. In practice, this would last for about 30-40 minutes before having to be topped up. I also purchased a hood with spare lenses, gauntlets and disposable overalls to wear during the blasting.

I anticipated the blasting would take three days to complete followed by a day to paint the fuselage but as ever it took longer for several reasons. After three days I had completed the initial blasting but there was a lot of paint that just wouldn't come off mainly around the cockpit area.

There was a mixture of paints, what looked like powder coating and some rubberised coating which I decided might come off better if I applied paint stripper. Another day was spent applying the stripper. The paint stripper used was SV-35/PMA formulated specifically for aviation paints (http://www.paint-stripper.co.uk/removall_paint_stripper_aviation.htm) and non-toxic. I left the stripper overnight to be cleaned off the next day. The results looked pretty good. I can recomend this product as it's easy to work with, strips the paint quickly (I was working in neart freezing conditions over winter so it should work very quickly in the summer months) and is easy to dispose of afterwards.

The approved method for removing the crud is by pressure washing with water which I don't have the equipment to do so instead it was brushed off with water, a messy and time consuming job. I then had to bead blast the whole fuselage again to clean up the remaining paint.

There were still odd spots of paint which hadn't been removed so another day was spent getting these off by using a craft knife blade followed by further bead blasting.

The finished product was cleaned with an air hose and tack cloths prior to painting.

In all the process took about 10 days spread over 4 weeks, but I did loose a week because the weather over here was so bad I couldn't get to the workshop. In hindsight I should have spent more time paint stripping and left the bead blasting just for the final finish but that's what this blog is about, helping you save time through my experiences. I think a realistic timetable for the job would have been 2 days to paint strip, 1 day to clean and 3 days to blast, still a lot of work but very satisfying once completed.

Next time, priming the fuselage.

Welding Work

During December I begun the preparation of the fuselage for it's eventual bead blasting and painting. There was no repair work necessary as the aircraft has never been crashed and there is no evidence of major corrosion but there are a number of other jobs that need to be completed.

The job list included:

1. Adding structural harness attachment points.


2. Welding on additional tabs to support the baggage sling, 'D' windows, hat shelf, floor boards and skylight.
   
3. Welding up and re drill the rear horizontal stabiliser mounting holes (to be re drilled).
   
4. Have my inspector sign of AD.2008-09-18 (Wing strut attachment bracket).

The plan was that if these jobs could be completed by Christmas, then I would grit blast and paint the fuselage in the period between Christmas and the New Year. Of course things don't always go as planned, but more of that later.

The first problem encountered was that the harness brackets that had been made (http://gbrih.blogspot.com/2009/11/harness-brackets.html) were flawed, the bolt couldn't be fitted because the fabric would interfere, so I had to make new ones to fit (see below). They were manufactured in the same way with the same spec material but now weld at an angle to give better clearance.

My friend David made four post supports from 4130 rod with an internal M6 thread. These were welded to the fuselage to provide additional support for the skylight.

The additional tabs were all welded on over a couple of evenings and I begun to plan the cleaning and painting of the fuselage frame.

The AD inspection was scheduled to be done once the fuselage had been bead blasted as this was purely a one off inspection for corrosion or cracks around the strut attachment fittings and it would mean I wouldn't have to remove paint at a later date.

I'll discuss stripping and repainting the fuselage in the next posting soon.

Fabric Removal - The Fuselage

Having removed the skylight, I started to tackle the main part of the fuselage by removing the fabric. The hope is that this will be removed in one piece (or as near as) so that I can use the fabric as a template later on when recovering. This may sound unnecessary but I forgot to measure up where the rudder cables exit the fuselage before cutting the fabric off, but I should still be able to get a fairly accurate measurement from the remains.

The tools required were just a Stanley knife, stout gloves and a mask. Razorback, being glass fiber is nasty stuff, and I had already got a rash from cutting small bits of fabric when transporting the aircraft to the workshop (once bitten, twice shy).

I started by cutting down the center of the fuselage top deck and then peeling the fabric off the sides, taking care not to scratch any of the tubes underneath with the knife blade.

An initial inspection of the tubing revealed very little , if any, corrosion on the upper longerons but the lower longerons are covered in dirt and grime accumulated over the years so I won't be able to see the corrosion until this is all removed and the tubes cleaned.

One interesting point I noticed was a kink in the starboard top longeron at one of the weld clusters which isn't mirrored on the port side. I've been told this may be due to weld 'shrinkage' or due to a deliberate outward curve on the tubes which will disappear when the fabric is shrunk on. If anyone else has any thoughts, let me know.




I wasn't surprised to find that spaces between the inner and outer fabric layers above the doors and around the D windows had been filled with an expanding foam; I already knew it was there, but I was keen to remove it and check that it hadn't trapped any moisture, potentially causing corrosion in the tubing.

So far it seems I may have gotten away with no corrosion in these areas.

What was surprising was the condition of the woodwork around the doors and skylight frame. Once it is all cleaned up a lot of it may be re-usable.

The D window frames are substantial and my plan at the moment is to replace them with smaller D windows. The top of the window will be in line with the top of the door, while the bottom edge will follow the line of the diagonal tube which runs from the back of the seat to the roof.

I still have the fabric around the port side door and window to remove and I need to remove the fabric from the bottom of the fuselage. When that is complete I can finish removing the boot cowl, clean up the fuselage tubes, inspect them for obvious corrosion and then look at stripping and inspecting the wings.