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Surface Preparation
Adequate surface preparation that removes the oxidized paint will help prevent the build up, the premature cracking of the pavement and will prolong the life of marking projects. So what constitutes "adequate"? Well, that's kind of "Catch-22". You'll be able to see evidence of "inadequate" surface preparation after the painting is done and the paint starts to peel worse than before, or cracks and starts to curl. It happens over time and is becoming a chronic problem on many runway surfaces around the country. A simple test of scratching a pen knife across the surface with downward pressure will give a clue to the strength of the paint bond. If it flakes or if you can pull up pieces, performing some sort of "surface" blasting (water blasting, shot blasting, sand blasting or grinding) prior to painting markings to remove 95% of loose and poorly bonded materials may begin to reverse the trend of paint chunks being FOD potential. FOD can be harmful not only to aircraft components but also to the airport itself from a liability standpoint. Falling under the definition of Surface Preparation is the removal of curing compound from new concrete pavement. Curing compound is sprayed onto the newly poured concrete and produces a moisture-resistant membrane film on concrete surfaces for thorough concrete hydration and strength development. It generally wears off the concrete over the course of one year, depending upon traffic. If paint is applied on top of curing compound, it will flake off as the membrane film sloughs off. Water blasting is a clean and effective way of removing the film. Another problem that is unique to airport pavements in warm, humid environments is green algae growth on markings. Algae is able to grow on pavement surfaces, and can seriously reduce the visibility of the markings. Airfields are particularly susceptible because the majority of the pavement receives very little or no traffic that would otherwise keep the algae at bay. Surface Preparation would include water blasting markings that have been covered with green algae.  As a word of caution, airport operators have a heightened concern about damage to pavement and fixtures on the airfield for good reason. Part of their job is to maintain a valuable asset: their pavements, as well as keep FOD to a minimum. Water blasters in the hands of the inexperienced can cause serious pavement damage in a hurry. Making sure that operators are well trained, the equipment is well maintained, and the right equipment is used for the job is extremely important. |
Although every specification for repainting a runway includes language
about surface preparation, most consider sweeping or blowing the existing markings to
remove loose dirt and paint as sufficient. But if you were painting your house and the
paint were flaking, would you sweep or blow the paint to prepare it for a new coat?
Probably not. Would you paint over mold or mildew? Not likely. So why are runway
pavements treated differently? Lots of reasons, but none of them justify the kind of
paint build up pictured. On runways, the problem is particularly troublesome because of
potential liability. If chunks of this paint get ingested into a jet engine, it might
cause damage, sometimes enough to cause premature wear of engine parts.
The marking shown to the right was a six-year-old, single coat of paint.
It could've been painted over without any problem, but it would have been the beginning
of the layering that causes markings to fail. With only 7,000 psi of high
pressure water blasting, the majority of the paint that was oxidized and poorly bonded
came up. The result was a surface much better prepared to receive a new coat of paint.
Doing proper surface preparation with water blasting, followed by vacuuming to remove the
resulting debris, will prolong the life of a paint project, give the airport owner much
more value for the dollars spent, and preserve the integrity of the pavement for a much longer
time.
Due to the width of runway markings, often 3+ feet wide, there is a
cohesive tension created across the surface of the marking. The paint bonds to itself,
as well as to the asphalt. In fact, the paint often bonds to the asphalt better than
the asphalt does to itself, sometimes causing premature cracking and pavement
deterioration, shown, right. Applying more paint to this kind of surface only worsens the problem.
The existing paint is fractured and has many openings for water to penetrate. A new
layer of paint will soon crack along the same fractures and water will continue to
undermine both paint and pavement.
