Question: My model is covered with POLYCOTE ECS and I have noticed that there are some sags in the covering. What is causing this? Can I fix it? What should I do?

Answer: It helps to understand how the sags develop before we get into tightening things up to get rid of the sags.

In low humidity air such as is found in the hot desert areas or in continental winter conditions where cold outside air is brought inside and heated to 22C ( 70F), the underlying structure is drying and shrinking. Many modelers feel that the sags are caused by heat and to a point they are correct because hot conditions are often dry and most definitely heat is the key to solving the problem. However, the real underlying cause is low humidity air drying the structure out and as the structure drys it gets lighter and smaller. POLYCOTE ECS will not shrink at normal room temperatures and you end up with,  in effect,  more covering on your model than you need!

The way to get rid of the sags is to shrink the POLYCOTE ECS with a heat gun and rub it down into the substrate with a soft cloth once it has shrunk. Before you start firing up your flame thrower, please note the following procedure... if you get the method down pat, you will get a good result and not have to do this more than 1-2 times... 3 times at most. However if you assume that POLYCOTE ECS is like Monokote or SolarFilm, you will make a hash of it and you will not be happy with the outcome. POLYCOTE ECS is polyester and the technique for shinking it is similar to that used for ORACOVER (ULTRACOTE).

Remember SSB! As in 

SECURE the perimeter.

SHRINK to fit

BOND to the structure.

First, SECURE the perimeter by sealing and bonding the seams, edges and around perimeters. Use a heat iron with sock set at around 250F and a soft cotton glove or sock. Heat the perimeter edge in stages, just warm up a small area and then  press the warm covering down firmly with the iron sock or a soft cotton cloth to bond the perimeter of the covering to the underlying substrate. Do not overheat or the edge will pull back as the material shrinks. Always rub along seams, not across the seam. Higher temperatures may assist with complex curved surfaces. Avoid using more heat than required to prevent bubbles from forming beneath the covering or edges pulling back as the material shrinks. Remember you are just SECUREing the perimeter at this point. Be patient and work systematically starting with a small out of the way area first to gain experience.

AFTER SECURING the perimeter, SHRINK the interior areas to fit, using a heat gun set at around 250K. Heat about 1 square foot of area at a time, shrink the covering to fit and then rub the warm covering down firmly with a soft cotton cloth to BOND the covering to the underlying substrate. Over open bays the POLYCOTE ECS will appear to first sag and then begin to tighten as you apply heat. Do this in stages and try repeatedly until the covering over the open bay is tight when cool. Don't overdo it... like may things, there is a technique to this that requies a bit of experience to develop. Go slowly one step at a time. Higher temperatures may assist with complex curved surfaces. DO NOT APPLY EXTENSIVE HEAT NEAR EDGES & SEAMS. Always practise on the bottom of a less noticable section first. Be patient and work systematically. You will likely only have to tighten POLYCOTE once or twice to accomodate any shrinkage of the airframe in dry hot conditions.

To prevent or at least minimize the occurence of sags do not leave your model in a closed car parked in the sun. Here is some information that may help you avoid this problem.