Login Form

Q-talk 89 - Tap Test & Repair

A new QBA member asked the Q-List "What are 'Tap Tests' and what do you do if you find something?"

Chris Walterson from Canada responded:

A tap test is when you take a coin and tap gently against the fiberglass of suspect areas. The areas with the foam attached to the glass will sound solid. The delaminated areas will sound hollow. If it's only small areas, drill a small hole and using a veterinary syringe inject some micro / epoxy mixture. Bigger areas may need to be treated differently. Remember, blue foam is susceptible to fuel and thinners of all types, even paint. The glass needs to be completely sealed even before priming. Most painters will wash off their project with "Wash and Wipe" thinner prior to painting. DO NOT DO THIS on a composite structure with blue foam. You can prep the composite surface for painting by using a little dish soap and lots of water as long as you allow the surface to dry thoroughly.

John ten Have from Australia responded:

Here are a couple of practical additions to Chris's advice. When you are tapping, mark the "change in sound" points on the surface with a felt pen (or pencil) - it will help you visualize the size and shape of any voids. Secondly, when you drill your holes, choose the best size for an airtight fit with the outside diameter of the syringe nozzle. It helps a great deal if you are able to drill an exit hole to relieve any pressure you introduce with the syringe. Inject from the low point and vent from the high point (if possible). If it is a big void, several holes may be required. Before you inject, use a bit of welding rod to measure the depth of the void through the hole(s).

Get and use several little syringes and drill out the nozzle as far as practical rather than using a big syringe - for two reasons. First, you can get away with a smaller hole and secondly, anything other than "cloudy resin" is diabolically difficult to inject. You can assist it by making sure the resin temperature is at the high end of the range but be very careful that you do not finish up with an exotherm due to warm resin / micro being injected into an insulated void. This could cause a bigger problem than you started with. If it starts to get hot, cold wet towels or crushed ice can sometimes save the day. Mix your micro in small batches. It also helps if you map out the directions of the laminates in the repair area so that when you drill your holes, you sever as few fibers as possible. Finally, repair the holes in the usual manner.

You can order a printed copy of Q-talk #89 by using the Q-talk Back Issue Order Page.