PTFE will not help the nuts become water tight, more than likely to cause a leak as it's a union fitting. A slight smidging of pipe jointing compound on the union face is more benificial.
Yes - use a paint sponge and get one with a long handle or use a small paint roller with a long thin handle. You can buy ones that are designed to be used to get down the back of radiators. Try Homebase or B&Q.