Dan Becker wrote:
>
> Query:  How to best remove peeling paint from a terne metal roof prior to
> repainting?
>
> Background:  I have a mostly standing seam, some flat seam terne metal roof
> on my 1926 domicile, installed between 1932-41.  Following the rehab
> project 4 years ago, "John the Butcher" painted the roof using the product
> of a local coatings manufacturer that had previously had a very good
> reputation.  As it turns out, my roof was painted at the very moment that
> they altered their formula [read: cheapened the stuff] before anyone knew
> what was going on.  Paint on the house exposed to ultraviolet and weather
> began massive and wholesale failure within 18 months.  The causes of this
> failure were very well described in Mike Edison's valuable and instructive
> recent tomes: my house is an Arts & Crafts period catalogue house, so the
> paint colors I selected are all heavily-pigmented earth tones...the pigment
> contents the vendor loaded into the base exceeded the CPVC, to quote Mike--
>
> >2. Binders, fillers, pigments
> >In pigmented coatings like paint, there is another critical element- - the
> >solid particles used to impart hiding power and color. Formulators refer
> >to the critical pigment volume concentration (CPVC) as the level of
> >pigment at which there is essentially just enough binder (resin) to hold
> >the dry film together. Some companies produce coatings with pigment
> >contents higher than the CPVC, so there is too little resin to completely
> >encapsulate and bind all the pigment particles. The upside is high
> >permeability due to imperfect films. The downside is high rates of
> >chalking and reduced moisture resistance.
>
> Here's my dilemma: how do I best get the failing stuff off now in order to
> repaint the roof?  I'm the general contractor, the subcontractor, the
> supplier, the site superintendent, the rigger, and the laborer for this
> one.  I'm reasonably confident that the specs will be adhered to.  Because
> the quality of what is on there is a known notoriously bad film, I really
> am loathe to leave any of it on the roof prior to repainting, lest the
> stuff fail beneath my new stuff.
>
> I am also loathe to think about the idea of having to manually wire brush
> the entire roof.  So...any means of effectively automating this process
> without using some mechanism that will puncture/damage the roof or me?
>
> Your suggestions are welcome.  I have this image of some wonder device with
> a rotating (not circular) drum of really stiff bristles, sorta like the
> main bristle drum of those old timey street sweepers that had the two round
> curb bristle discs out front, the rotating drum in the middle, and the
> single tire for steering in the back.  Make a clean sweep of the roof.  I
> can't image trying to do any kind of dry abrasive blasting to strip the
> roof, nor does trying to do a pressure wash sound too smart.
>
> ____________________________________________
> Dan Becker
> Executive Director, Raleigh Historic Districts Commission
>
> [log in to unmask]
===============================


Dan:

Lots of times its the surface preparation and the method of applying the
primer that are most important parts of painting metal roofs. I would
assume that was the problem more that the manufactures changing
ingredients. Also, do you have trees near or over hanging the roof. What
type of trees? Being a good southern boy are your surrounded by pines or
tulip poplars that sometimes do not know when to quit with the pollen in
the spring. Was the roof last painted during pollen season along with
less than serious preparation? Trees also allow moisture to be a more
active agent.

Our procedure for metal roofs is usually:

First: Repair areas of damage. Different types of deterioration / damage
require different repair techniques.

Second: Power wash the roof with a good low volume / high pressure rig.
Always work from the top down. Always have the business end of the power
washer pointing down the roof, not up the roof. (Work with the design
details, not against them.) A good power washer with a narrow fan tip
will peel the loose paint right off and even can do a number on well
adhered paint.

Be sure to set up cloth drop cloths to catch the flying debris. Do not
use plastic or rubber backed drops. Cloth allows water to pass through
and allows the paint chips to cling to the cloth. Some paint chips will
fly. Make clean-up an important task.

Third: Scrape and wire brush the surface as required.

Forth: Just before priming, lightly sand and wipe with a tack rag.

Fifth:  The primer is the most important paint layer. --- Take a good
bristle brush and cut the bristles so they a only about an inch long.
Apply the primer with the modified brush in a scrubbing action. This
will help force the primer into the porous surface of the metal. Priming
should wear you out, if it does not wear you out, then you did not scrub
hard enough. (Pretend you are in your aerobics class - "Come on now,
scrub - scrub, work it out - scrub - scrub - do it right - three - four
- make it hurt - three - four. Left hand - scrub - scrub - Right hand -
scrub - scrub. Can you feel the burn - scrub - scrub - work it out -
scrub - scrub")

Forget the magic machine or the magic goop. Just put the word "LABOR"
back into the process.

Use a good quality oil based metal primer - scrub - scrub.

Six: Use a good quality acrylic paint for the top coats. (Michael - -
comments or recommendations on top coats.)

Seventh: Next time realize that you get what you pay for - usually. So
stay away from "John the Butcher".  Pay for the person that has invested
in themselves, that knows their value and has honor and pride in their
work.

Bryan the Butcher
of
Dell Corporation