Q-talk 89 - Finishing
- Category: Q-Talk Articles
- Published: Wednesday, 23 December 2009 16:24
- Written by John ten Have - Australia
- Hits: 1393
For those who have yet to start the finishing process of your Q, the following might help:
1. Everyone has only so much sanding in them, so make it count and minimize the work required.
2. Use the right materials, cheap abrasives are no bargain. Use West system epoxy finishing materials. They work and they are softer and cheaper than the structural epoxies.
3. Start finishing from the time you begin construction. It seems to elude some that putting 0.020" of glass over "corrugated surfaces" will not automatically transform them into flat surfaces. Foam is much easier to sand than glass, so sand it and get the core dead right prior to glassing.
4. Use several size sanding blocks and use them intelligently. Sawing back and forth on a curved surface will not ensure it stays curved . +-45 degree to leading edges works in most places
5. Ensure you know the difference between flat and smooth. A billiard ball is smooth but it is not flat.
6. Peel ply everything. That way there is a dramatic reduction in sanding the hard surfaces, no jagged edges as well as far less structural damage. There is also much less filler shrinkage, not to mention a lot less filler.
7. Stop sanding the moment you hit glass.
8. Don't guess, map the lows and fill them first. Use straight edges of varying lengths and plot the fill and sand cycles. Learn to color sand. Get familiar with and use the tools plasterers use, troweling on micro so it is smooth is much, much smarter than icing a cake and sanding.
9. Go from the general to the specific - i.e. get it the right shape first. 60 - 80 grit will really shift micro in a hurry. Do not worry about the surface finish until the shape is right. Know what the right shape is before you start. Learn the difference between the right shape with a rough finish and the wrong shape with a smooth finish.
10. Learn how to form even gaps.
11. When the shape is right, sand with 100 grit.
12. Choose and use a finishing system which has compatible chemistry from top to substrate inclusive. Get the application notes for the product and follow them.
Final thought: Finishing is the last thing you do but it is the first thing that everyone sees. It is carried out when the builder is at a time, energy, resource and emotional ebb. It is no wonder there are so many very sound aircraft which look as though they have been painted by Mr. Bean. It need not be so. Aircraft are long time built.
(When questioned about the use of peel ply on everything, John responded with the following justification.)
Resin starvation has nothing to do with peel plying. It has everything to do with laminating expertise in preparation, execution, inspection and standards that are set. With normal wet layups, you can never get glass to resin ratios close to those possible with competent vacuuming of wet layups and you are very hard pressed to get close the figures which routinely emerge from pressure/ prepreg/ oven curing.
When you wet layup glass and then squeegee too much, your little bumps of glass compress down under the blade of the squeegee, and then spring back up again, sucking in air. If you do the same thing with a peel ply, the site is sealed and air re entrapment is much less likely, particularly if you start with a slightly resin rich layup. The peel ply stays down, consolidates the layup and smoothes the surface. With peel ply you will do better than wet layups - but that is only part of the process- the wing ain't finished 'till its finished!
Don't take my word for it, do the following experiment.
Get two sheets of alloy, wax/pva them, and do a layup of 3 BID 8"x8". Peel ply one with white (no perforated) Dacron fabric and make sure the temperature is at least in the middle of the recommended working temperature range. Squeegee it until you can no longer see a ridge form when you wipe it across the peel ply surface. You will know when you get too lean and the excess can be worked back into any areas where you overdo it. Push the resin off in all directions. Do an equivalent layup following the conventional "wisdom". Let them both cure. Cut a 6"x6" square from each and weigh them.
Next sand them until they are both completely white and time the process. Then perform the finishing process through to the top coat, timing each operation. Weigh them again and then decide which is the smart way to proceed.
Here are additional reasons why you should use Peel Ply.
1. It enables a much leaner layup i.e. much more resin can be removed without pulling air into the laminate. The layup is therefore lighter.
2. It forces the fibers to lie flatter. This improves the mechanical performance (less kinking is better).
3. By consolidating the fiber bundles and reducing the peaks and troughs, much less filler is required and far less sanding (read fiber damage) is needed before subsequent layups and finishing.
4. If and when you need to sand, the surface is much easier to sand to the state required ie a uniform scratched surface rather than trying to sand down into the troughs
5. Sanding of "natural weave will knock the peaks off first which will result in much deeper bundle damage than if the fibers were all collinear. ( so much so that smart designers make allowances for up to half to the top skin to be removed in the sanding process. Unfortunately other designers do not)
6. It helps support vertical tapes if they are laid up on peel ply first and then carried to the job.
For the reasons stated above, the layup is not heavier, it is stronger and will remain so after the finishing process.
Peel plying can be considered the poor man's vacuum bagging.
You can order a printed copy of Q-talk #89 by using the Q-talk Back Issue Order Page.