Original Air Date: 07/25/26 Episode Number: 469

Episode Summary

A listener question about painting aluminum siding kicks off the show, and Dan splits the answer across two segments with a detour in the middle: a full conversation on how to identify and safely clean biological growth off the exterior of your home -- algae, mold, mildew, moss, and lichen. The aluminum siding answer wraps up at the end with product recommendations and a strong case for always doing two coats. Moss and lichen get their own episode next week.

Timestamps

• [00:00] -- Show Kickoff
• [00:30] -- Listener Question: Aluminum Siding
• [01:44] -- Oxidation and Prep Basics
• [03:58] -- Wash, Scrub, Rinse
• [05:42] -- Hand Test and Inspection
• [07:21] -- Primer or Not
• [08:11] -- Switch to Algae Topic
• [08:53] -- Brick Recap and What Triggered This
• [12:20] -- What Growth Is It?
• [13:32] -- Algae, Moss, Lichen, Mold
• [16:16] -- Efflorescence Sidebar
• [17:29] -- Moisture Causes First
• [19:01] -- Cleaning Safely Tease
• [20:01] -- Moss vs. Mold Distinction
• [21:00] -- Golden Rule: Gentle Cleaning
• [22:47] -- Pick the Right Cleaner
• [25:50] -- Prep Before Repainting
• [27:09] -- Hose, Not Pressure Washer
• [29:53] -- Why Not Straight Bleach
• [31:22] -- Protect Plants from Runoff
• [32:41] -- Fix the Moisture Root Cause
• [33:56] -- Aluminum Siding Paint Picks
• [34:45] -- Two Coats Really Matters
• [36:33] -- Best Benjamin Moore Options
• [38:54] -- Temperature and Wrap Up

Listener Question: Aluminum Siding [00:30]

A listener wrote in asking about painting their tan aluminum-sided house a darker blue. They wanted to know what prep was involved, whether one-coat coverage was possible, and what products to use.

Dan splits the answer into two segments. The short version on the one-coat question: two coats, always. The longer version on everything else follows below.

Oxidation and Prep Basics [01:44]

The single most important thing to understand about painting aluminum siding is oxidation. If you've ever rubbed your hand across older aluminum and ended up with white chalky powder on your palm, that's the original factory finish breaking down from years of sun, rain, and weather. That chalk has to come off before any paint goes on.

Paint bonds to a surface. If that surface is a loose layer of chalk sitting on top of old factory finish, the paint is bonding to the chalk -- not to the siding. When the chalk lets go, the new paint goes with it. This is the most common reason aluminum siding paint jobs fail, and it's entirely avoidable.

A pressure washer alone won't get you there. It'll knock off dirt and cobwebs, but it doesn't automatically remove oxidation. Dan compares it to washing your car with only a pressure washer after an entire Michigan summer of construction zones and dirt roads -- you'll get some of it, but you won't get all of it.

Wash, Scrub, Rinse [03:58]

Start with a good exterior house cleaner that's built to break down dirt, grime, mildew, and oxidized residue. Apply it, and then physically scrub the surface -- a soft-bristle siding brush on an extension pole makes this manageable. Work in sections so you can scrub and rinse before the cleaner dries on the siding.

You can use a pressure washer for rinsing, but keep the pressure reasonable and always direct the spray downward the way rain would naturally hit the siding. Shooting water upward beneath the panel laps can drive moisture into the wall. The pressure washer is a rinsing and help-with-cleaning tool. The scrubbing still has to happen.

Hand Test and Inspection [05:42]

Once the siding is dry, perform the hand test: rub your palm firmly across the surface. If it comes away clean, you're good to move on. If there's still a chalky residue on your hand, keep cleaning. The surface doesn't need to be microscopically perfect, but you should not be able to casually wipe your hand across it and look like you've been handling powdered sugar.

While you're working your way around the house, inspect as you go. Look for peeling paint, corrosion, bare aluminum, dents, damaged panels, and failed caulk around windows, doors, and penetrations. Bare or corroded areas may need spot priming before finish paint -- bring photos into RepcoLite and they'll point you toward the right primer. Loose paint should be scraped away and edges feather-sanded so they don't telegraph through the new finish.

Primer or Not [07:21]

If the original factory finish is clean, intact, and firmly bonded, you do not need to prime the whole house. No matter what you read online. A high-quality exterior acrylic straight over a clean, sound factory finish is the right call. Primer is for bare spots and problem areas only.

Biological Growth: What Are You Actually Looking At? [12:20]

Dan took a walk around his own house after mentioning offhand on last week's show that brick doesn't need much maintenance -- "every now and then you clean the algae off." That comment nagged at him all week until he went and looked. His back wall, under two old maple trees, was covered in green growth on both the brick and the vinyl siding on the addition. This week's middle segment covers what to do about it.

Before you clean anything, figure out what you're actually dealing with. The growth type matters, and the surface it's on matters just as much.

Algae is the most common. Thin, flat, greenish film, usually on shaded or damp areas. Feels slick when wet. Spreads across the surface rather than growing in clumps.

Moss is thicker and softer -- you know what it looks like. It holds moisture against the wall, which creates problems depending on what the wall is made of. On vinyl it's mainly a staining and maintenance issue since vinyl doesn't rot, but on wood siding or shingles it can contribute to paint peeling, decay, and eventually rot. On brick, stucco, or stone it keeps mortar joints and damaged areas wetter longer than they should be.

Lichen is crusty and firmly attached -- it can be gray, yellow, pale green, orange, or black. It's the toughest of the group to remove because it grips the surface aggressively. On soft brick or old mortar, scraping it off can pull part of the surface along with it. Lichen on roofs gets its own conversation next week.

Mold and mildew tend to appear darker, blotchy, fuzzy, or powdery.

Efflorescence -- the white, fuzzy stuff sometimes seen on brick or concrete -- looks biological but isn't. It's salt deposits left behind by moisture moving through the masonry. Address the moisture source; don't just clean it off and move on.

Dan is covering algae, mold, and mildew this week. Moss and lichen get their own episode next week, including how to deal with them on the roof.

Fix the Moisture Problem First [17:29]

Cleaning is not a permanent fix if you don't address why the growth is there. Moisture is the cause of all of it.

Before mixing any cleaner, look at what's creating the damp conditions: overflowing gutters, a downspout dumping water next to the wall, a sprinkler hitting the siding, shrubs growing too tight against the house and blocking airflow, mulch or soil piled high against the brick, or heavy shade from tree canopy keeping the wall from drying out. If you clean the wall and leave the moisture problem in place, the growth is going to come back -- sometimes pretty quickly.

Cleaning Safely: JoeMax [22:47]

Start with the gentlest method that will get the job done. Don't reach for the strongest chemical available or fire up the pressure washer first.

Inspect the surface before you do anything. On brick, check the mortar joints -- is it solid or loose and powdery? On siding, look for loose panels, open caulk joints, peeling paint, or damaged edges. The condition of the surface determines how carefully you have to work.

Also confirm you're actually dealing with biological growth and not just dirt or pollen. If it's just a surface film of grime, mild dish soap, a scrub brush, and a hose will probably handle it without any specialty product.

If it's actual biological growth, the product Dan recommends is JoeMax House Cleaner and Mildew Killer. It's a concentrate that combines a mildewcide, bleach activator, and detergents. You mix it with household bleach and water. It kills mold and mildew and lifts dirt and staining at the same time, and it's rated for virtually every surface on the outside of a house -- vinyl, aluminum, painted or stained wood, trim, fences, stucco, concrete, and brick. One product covers everything. For Dan's situation with both brick and vinyl on the same house, that matters.

Using it is straightforward: mix per the label, spray on, give it about five minutes to work, and rinse off. Most smooth surfaces won't need scrubbing. Textured surfaces like brick may need a light scrub. Mix only what you'll use in the next few hours -- once bleach is added, it's only good for a short time and can't be saved.

Why Not Just Use Straight Bleach? [29:53]

Bleach is not a great cleaner on its own. It beads up and runs off rather than wetting the surface and lifting residue. It also flashes off quickly -- it can lighten a mildew stain and evaporate before it actually kills the growth, so the problem comes right back. JoeMax has detergents that let it wet the surface, carry the contamination away, and stay active long enough to do the job properly.

Hose, Not Pressure Washer [27:09]

For the actual cleaning, a garden hose with a soft-bristle brush is usually the right tool. Natural bristle on brick, soft nylon or cloth on vinyl. High-pressure washing on brick can erode weak mortar, etch the surface, and drive water into the wall. On vinyl, water sprayed upward beneath the laps can get inside -- vinyl is built to shed rain coming downward, not a concentrated jet coming from below.

Protect Your Plants [31:22]

Don't skip this. Water plants thoroughly before you start -- wet the leaves and soak the soil so the roots aren't sitting dry and ready to absorb runoff. Cover anything especially delicate while you apply the cleaner, but don't leave plastic sheeting over plants in direct sun or you'll cook them fast. Get it off as soon as you can. Work on a calm day to minimize drift. Keep a second hose nearby to rinse accidental overspray immediately. Don't let runoff pool at plant bases or drain into a storm drain, pond, stream, or rain garden.

If You're Repainting, You Still Have to Kill It [25:50]

If the surface you're cleaning is going to be painted afterward, don't skip the mildewcide step. A mildewcide in your paint prevents new mold from forming -- it does not stop existing growth from working its way back through from underneath. Clean it, kill it, rinse it thoroughly, let it dry completely, then prime and paint. JoeMax handles all the prep steps you need before paint goes on.

Aluminum Siding Paint Picks [36:33]

Back to the listener question. For aluminum siding with clean, sound factory finish, you want a high-quality exterior acrylic. Two coats, no exceptions.

The first coat hides the original color. After one coat you might think you're done -- and on a calm day standing close to the wall, it might look fine. But in afternoon sun, or when you step back, thin areas and lap marks and variations between brushed and rolled sections become obvious. The second coat evens out the color, improves sheen uniformity, and -- most importantly -- builds the film thickness the paint was designed to have. Durability comes from having enough coating on the surface. One coat might change the color; two coats creates the protective film.

Benjamin Moore Aura Exterior is the premium recommendation, especially for a significant color change like tan to a rich dark blue. It hides exceptionally well, holds color long-term, and handles strong, deep colors better than most products.

Benjamin Moore Regal Select Exterior is a high-quality option with excellent application properties and long-term durability. A strong all-around choice for a full repaint.

Benjamin Moore Element Guard is worth knowing about if there's any weather uncertainty. It only needs about an hour before it can handle rain or heavy dew, which matters in Michigan's shoulder seasons when the forecast can change while you're opening the can. Less relevant in peak summer, but a smart pick in spring or fall.

All three use Benjamin Moore's Gen X colorant system, which is formulated to work with the paint rather than weakening the base the way traditional universal colorants can. Better hiding, better durability, stronger color performance in deep tones.

RepcoLite Endura uses a similar premium colorant approach and will likely offer a price advantage while still delivering strong performance.

One final note on aluminum siding specifically: watch the surface temperature, not just the air temperature. Aluminum, especially in a darker original color, heats up fast in direct sun. The temperature range on the can or the tech data sheet applies to the surface you're painting on, not the air around it. Work within that range or you risk adhesion and application problems.

Coming Next Week

Moss and lichen -- how to remove them safely from brick, wood, stucco, stone, and roofing without damaging the surface. Dan promises it's better info than it sounds.

Products Mentioned

• JoeMax House Cleaner and Mildew Killer -- Concentrate; mix with bleach and water; rated for vinyl, aluminum, wood, brick, stucco, concrete; available at RepcoLite
• Benjamin Moore Aura Exterior -- Premium exterior acrylic; excellent for deep color changes; available at RepcoLite
• Benjamin Moore Regal Select Exterior -- High-quality, durable exterior acrylic; available at RepcoLite
• Benjamin Moore Element Guard -- Rain-ready in about an hour; great for unpredictable weather; available at RepcoLite
• RepcoLite Endura -- Premium exterior paint with strong colorants; available at RepcoLite
• Roma Bio Limewash / Roma Bio Mineral Paint -- Mentioned briefly as the right products for painting brick (covered in detail last week); available at RepcoLite

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